This seems like you have a issue with the mirror you are using or something else is wrong.
You should try to find out why and when it is a problem on your end try to fix it.
pacman-mirrors
Will output your current mirrors
sudo pacman-mirrors --country all --api --protocols all --set-branch stable && sudo pacman -Syu
Will reset the values to the default but might be a to large a group of countries so select the mirrors you want with this command:
As you may have seen, a number of helpful posts mentioned pacman-mirrors.
An update will never be successful, no matter how many times you try, if the mirrors in your mirror pool are not available or in-sync. You can verify the status of your mirrors by using the command pacman-mirrors and/or viewing the webpage https://repo.manjaro.org/.
You can reorder your current mirror pool (–fasttrack) or change the mirrors in your mirror pool (–country, --continent).
You can read more about pacman-mirrors in the Manjaro wiki documentation, and also man pacman-mirrors.