only ideas i have is 1. check both power cables going from the video card to psu and make sure proper psu rails are used when connecting. each rail puts out diff power. 2. also could be that either though your PSU is rated at 850w if its a cheap brand you may not be getting close to that, 3. your PSU could be on the verge of failing which would cause shutdowns etc. 4. vid card could be defective. sorry if this is vague but these are the most common things that happen with this sort of problem.