I would probably first look at the connections for the PSU and make sure the Graphics card is seated corrected and all of its connections are secure.
I suppose you have reinstalled the Graphics driver?
What resolution are you running?
I suppose running the Windows Experience Index would cause the system to error? If not, what number do you show for the index on Video?
Do you ever get a warning from Windows, after gaming, about the Video settings needing to be changed?