Well the first thing you need to do is check the heatsink make sure it is tight on its pins in the motherboard it sounds to me that it most likely isn't sitting properly on the CPU. If it wasn't installed correctly it could have become loose, those pins are really hard to get in the right position sometimes and over time the heatsink will slip further and further off the heatsink. Also take the heatsink totally off clean the old thermal compound off with rubbing alcohol and a q-tip and wipe it clean with a coffee filter or something that is lint free and reapply some new thermal compound. Besides that make sure all fans are running and clean all dust out of heatsinks and computer itself.
After all that and still having trouble check in the bios if you can and make sure all the voltages are set right vcore should not be above 1.36 volts.