With such low info about your problem is hard to help. But to give a try…
As long your pc doesn’t shutdown and only freeze, it’s not CPU related or heat problem.
My guess is either (most probably) RAM problem (fault or limited RAM, freezing when trying to swap), or power management problem.
If I had the same issue, my steps would be:
- Open gnome-system-monitor and xsensors probably looking at ACPI and on a terminal journalctl -f and try to reproduce the issue and understand what’s wrong, for example eating RAM and swapping, too many heat, throttling etc.
- After freeze, restart and see at log from previous boot:
journalctl -b -1 or gnome-system-log and look at the end of the log (where your pc froze) for specific errors.
How would I run a memtest?
You can use either your bios memtest or if it’s doesn’t have then try booting with (usb or dvd) live-boot from manjaro and select memtest.
Also when to are looking for help read this first