*Soz to bring this thread back up*
But I have started to have a very similar problem. I thought it may have been my power supply, but then I realised it has only started to happen after I overclocked my CPU (it was already overclocked, but I decided to overclock it more). I have now just underclocked my CPU back down to what it was originally and it seems fine. I'm gonna see how it goes for a few hours though but I didnt actully know that the CPU would make it turn off (seening as it really didnt overheat, was only reaching about 40-45c).
I hope this helps at all.
Jamie.