I'm thinking this might be a thermal issue; possibly the
CPU heatsink isn't seated properly, or maybe the
PSU is overheating. Are any of your fans blocked/busted?
First thing I would do is reseat the heatsink, removing all Thermal Interface Material and applying new
TIM. Make sure you seat the heatsink properly, with the same amount of torque on all 4 heatsink mounting screws.