This should in trouble shooting or hardware section.
Is this is a prescott which heats a lot? If yes, then this might be the prob.
There have been problems with 7900GT and oc is even worse, this is likely to the problem as well.
A corrupt comp, with lot of trash in it due to lack of regular formatting if required(say once every 3-4 months or even 6 months on the higher side), might be the prob.
Check comp temps and room temps, gpu temps, cpu temps etc.
On what settings can you run the game fine with an average of 45-60fps or higher?