Did you or the computer shop you took this to every try another power supply? That Ultra PSU(despite having a lifetime warranty which they don't honor btw) are cheap piles. its a 750w psu but only has 45a on the 12v rail. A good 750w PSU like the Corsair HX750 has 62a. That's where I'd start. Unfortunately the error you are getting can be cause by numerous issues both hardware and software. By doing a clean reinstall(im assuming here your reinstall was clean) you eliminated the software side of it. Usually on the hardware side there are 2 main culprits he power supply not supplying enough power, or the video card itself. I have seen a bad monitor cable cause this issue as well, I caught that one by pure luck.