It coult be a dodgy connection inside the computer (PCI slot), overheating casuing the GPU to become unsoldered for a second, underpowering due to a secondary power supply to the gfx card not bing connected, or perminant damage to the gfx card like a dry joint or cracked pcb track. Alternantively it could just be an incompatability between your monitor and the gfx card; have you tried lowering the resolution and also trying it on a different monitor (such as your tv maybe if you dont have another pc monitor)?