Take out the graphics card and use onboard for now, see if the problem persists. It's a pity you don't have a different graphics card to test with :/
Also, long shot but could also be a potential issue with your power supply. Dirty power can make graphics cards perform strangely, don't suppose you have a spare power supply by any chance?
Also, long shot but could also be a potential issue with your power supply. Dirty power can make graphics cards perform strangely, don't suppose you have a spare power supply by any chance?