

The only two things I can currently think of are:
- Shut down the PC and try inverting the slot your monitors are plugged in in the graphic card
- Right click on the Desktop -> Swap the primary Display -> Apply -> Swap it back
Honestly I’m not sure if it will help but I don’t have other ideas aside from purging the configuration, which is probably not the solution and unwarrented.
Another thing to try would be a search on the KDE bugtracker.
Hangs, reboots or does it turn off? Depending on that it could be a plethora of things.
Do you get any error prompts? Any red alerts when restarting?
Did you check that the monitor works steadily on another system/OS?
Did you try another DE, even on a live usb?
Is the HDD/SSD healthy?
Are all the fans working? Is it the thermal protection?
Is the PSU healthy and or the power connection damaged?
Does your system have a centralized logging like
journalctl
or can you reach the single log files to check and add more information?It could literally be anything, even aliens.