Well, at this point it sounds like it is one of two things: the Power Supply or the Video Card.
Ultra PSUs are mediocre at best, however 750W leaves a lot of headroom so that should be OK.
So, the graphics card may be dying. Why? Who knows, it might not have taken to the unlocked shaders well. I'd recommend starting the RMA process, and return it if you don't get any other feedback here.