A couple of thoughts come to mind. First of all, did you use Artic Silver 5 for your thermal paste between your heatsink and CPU. If you did, how much or little did you put on there, because it sounds like overheating or a poor thermal solution. If this were mine, I would first clean off the old thermal paste and put artic silver on there again.
Applying Artic Silver Thermal Paste
Also, if your CPU is not clamped down tightly (or screwed if that is the case), then you will see the shutdowns, so make sure it is clamped down tightly and one leg is not loose when you fire it up.