I guess that could just be a lower ambient temp in the room doing that. Do u have any more heat sink paste? is it possible for u to take the hsf off clean it, then reapply a very small amount again to double check its all on ok. I cant think what else it could be, the only othjer thing is if the psu u tried is powerful enough, what wattage is the psu u tried?
Do u have an onboard gfx? if so u cud take ur gfx out if u have one to rule out a damaged gpu.