So what happens when it turns off? Is it shutting gown at idle or under load? If the problem is under load how long can it be loaded before it shuts down? Immediately, 10 minutes, etc.. What bios are you running on your motherboard?
I would have thought for sure it would be power supply so I don't really know what's going on. In this case you could do what I'd usually do which is try everything. Disconnect all your drives except your OS drive. Remove all but 1 stick of ram. Take your 260gtx back down to stock speeds (if it's overclocked). Just start trying shit But definitely check your voltages.
Btw, there are a lot of good threads over at xtreme systems about q6600 overlocking. I'd search through them to get an idea of what bios settings you should be using.