Failure modes that come to mind:

-Power line voltage drop.
-Power line voltage surge triggering surge arrestor or over-voltage limit in power supply.
-Loose connector: at wall socket, power cord to PC, adapter to monitor, power supply to graphics adapter.
-Graphics adapter not seated properly in slot.
-Failing/overheated graphics adapter, PC or monitor power supply. Dust buildup or failed fan most likely cause of overheating.


Most of these can be ruled in/out in a couple of seconds. When was the last time the PC got a good cleaning? I've seen a lot of unreliable computers that just needed the dust cleared. It is a really simple job.