Have you tried reseating the cpu heatsink? It may be overheating, I've seen that a lot. You may also want to check the TIM and reapply if what you have now is old or starting to dry out.
what is TIM? and how do i reapply it?
Thermal Interface Material. Make sure your heatsink is properly seated also, as those stock Intel ones can be a real bit** to get on right. To reapply TIM , wipe if off with some alcohol, and get some Arctic Silver 5 to replace it. It is definitely overheating.