If this issue appears without reboot, then check journalctl -p err -b 0 --no-page that will show some error messages.
I have no issue with Intellij IDEA Ultimate Edition (from the official website) on KDE Plasma 5.25 Wayland on AMD system. It looks smooth to me.
But I use my customize KDE theme very close to “vanilla” (a uncustomized version) KDE theme. I’m not fan of any heavy custom theme e.g. third-party theme app that would not be compatible with KDE 5.25 or newer in the future.
$ journalctl -p err -b 0 --no-page
сен 30 04:13:58 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:13:58 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:13:58 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:14:00 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:14:00 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:14:00 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:14:00 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:14:00 baroness-b450mh kernel: kvm: support for 'kvm_amd' disabled by bios
сен 30 04:14:03 baroness-b450mh kded5[596]: org.kde.plasma.dataengine.geolocation: error: "Неизвестный узел location.services.mozilla.com: Узел не найден"
сен 30 04:14:03 baroness-b450mh kded5[596]: org.kde.plasma.dataengine.geolocation: error: "Неизвестный узел location.services.mozilla.com: Узел не найден"
сен 30 04:14:07 baroness-b450mh wpa_supplicant[659]: bgscan simple: Failed to enable signal strength monitoring
сен 30 04:14:29 baroness-b450mh pulseaudio[922]: GetManagedObjects() failed: org.freedesktop.DBus.Error.NoReply: Did not receive a reply. Possible causes include: the remote application did not send a reply, the message bus security policy blocked the reply, the reply timeout expired, or the network connection was broken.
The error messages might not be related to your issue?
I don’t know
Looks like at the new user everything works okay
That was started after stable upgrade, but I don’t know how to downgrade kde because I don’t see certain package that I need to downgrade.
It seems something is wrong in your (old) home user.
If that does not help, you can switch to the new user and copy your some data from the broken user to this new user. Then delete the broken user.
i had random freezes because of chrome based browsers.
there are some features need to be disabled in these type of browsers to avoid random freezes
I don’t have any, I use Firefox
No, I still see that issue sometimes. There are no anything different at journalctl -p err -b 0 --no-page.
Can I downgrade KDE to 2.24? Before 2.25.5 everything was perfect.
If you’ve backed up everything…and TimeShift is working properly…you can, then, probably roll back 
And, yes, Plasma 2.24 was impeccable…
Hello. How do I downgrade KDE to 2.24? I know about downgrade tool but I don’t know what packages I need to select.
I want to downgrade because of freezes on 2.25.5 version. It’s hard to work now, I even can’t switch windows as normal.
I don’t have that old timeshift snapshots.
Short answer: You can’t.
Longer answer: Create a new user, see if problems persist. If they don’t, you just have to selectively clean your ~/.config and perhaps ~/.cache to get rid of old plasma settings.
Same here, I forgot to backup everything(but it’s done now…a bit late though).
So I dunno take a thumb drive(or other) and try to reinstall with your ISO. If it’s Plasma version 2.24 you might be saved(hence, do NOT update your system).
Otherwise, you’ll just have to ride this out till 5.26 ends up in Stable Branch. And, that’s not for tomorrow: 5.26 is previewed for October 6, and not before… So Testing Branch are going to see what gives before anything hits Stable. You are forewarned 
The problem is that sometimes I can work for several hours without problems until the freezes begin. After creating a new user, I can’t be sure that this problem won’t come back to me
I usually create a timeshift snapshot before updating and delete it after a while if everything is fine. And everything was really normal, the problems did not start right away
Maybe now I just have to be patient and wait for the release of the fix. I can’t believe the Manjaro team merged this version into the stable branch, I’m not the only one with problems
There is little to no chance there will be a fix for “random freezing”. This is what happens over time. If you just installed your system with 5.25, then it might happen when you are on 5.29. And you can’t even be sure it’s (solely) Plasma’s fault. That’s how it is with “random freezes”, sadly - near impossible to pinpoint the cause.
I have been using Manjaro for several years. The last time I reinstalled the system was July 15th, updating every time a stable update came out. Windows works perfectly, I do not see any problems. I didn’t change the system settings, I didn’t install anything new, I literally didn’t change anything - after the update, friezes appeared and these are not only my problems - other users also complain about it
Therefore, it seems to me that something was broken in 2.25, which is why many users began to have similar problems. And I hope KDE will release a fix. I can try reinstalling Manjaro, but I’m afraid I’ll be wasting my time because nothing will change
I’m sad that it’s not possible to stay on the LTS version of KDE. Everything about her was perfect
Or some old settings have become incompatible with new plasma version (I agree this is a bit of a stretch if you reinstalled not even 3 months ago, but still… Also, why are you reinstalling? 
). Hence, try a new user, or delete relevant plasma config files and start fresh.
It seems to me you have already answered your question.
If the new user works OK, then the problem must be in your current or old user settings. Finding out what is different, or even ditching the old user would probably go a long way to solving your problem.
Continuing to operate in the old user, in the hope that things will turn out different this time seems, to me, futile.
I changed pacman.conf to ignore plasma upgrade and this is the list of ignored packages, may help you to know which ones you have to downgrade:
advertencia: bluedevil: ignorando la actualización del paquete (1:5.24.6-1 => 1:5.25.5-1)
advertencia: breeze: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: breeze-gtk: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kactivitymanagerd: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kde-cli-tools: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kde-gtk-config: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kdecoration: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kdeplasma-addons: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kgamma5: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: khotkeys: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kinfocenter: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kmenuedit: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kscreen: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kscreenlocker: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: ksshaskpass: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: ksystemstats: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kwallet-pam: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: kwayland-integration: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-2)
advertencia: ignorando el remplazo del paquete (kwayland-server-5.24.6-1 => kwin-5.25.5-2)
advertencia: kwin: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-2)
advertencia: kwrited: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: layer-shell-qt: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-2)
advertencia: libkscreen: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: libksysguard: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: milou: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: oxygen: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: plasma-browser-integration: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: plasma-desktop: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: plasma-integration: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-2)
advertencia: plasma-nm: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: plasma-pa: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: plasma-systemmonitor: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: plasma-workspace: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-4)
advertencia: plasma-workspace-wallpapers: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: polkit-kde-agent: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: powerdevil: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: sddm-kcm: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)
advertencia: systemsettings: ignorando la actualización del paquete (5.24.6-3 => 5.25.5-1)
advertencia: xdg-desktop-portal-kde: ignorando la actualización del paquete (5.24.6-1 => 5.25.5-1)