Then personally i would open her up, clean the hsf well with cotton wool buds (you know the ones for your ears) clean the gpu especially the heatsink and generally around any other case fans,psu fans and any dust build up areas. Even if this is not your ultimate problem, you'll still be doing your pc a brilliant service. Although i still have a feeling this will kill your problems. Good luck and be sure not to touch any components without at least resting your bare arms on the metal sides of the case at the same time xx
Max.
xx ESD