The media center pc had started to power down in the middle of watching netflix or playing a game for a while.
Fired up a temperature monitor and sure enough, if the processors were above around 80% utilization the core temps would steadily rise until the machine powered down to protect itself.
I maxed out all the fans and even made a cardboard wind tunnel to suck more air across the heat sink, but the temps still would rise under load.
Finally took the heat sink off and looked at the thermal grease, it was dry and cracked over half of the chip! I must have put too little on last time I took the processor out.
Cleaned off the CPU and applied a more liberal amount of thermal grease, put everything back together and started a game that had made it overheat previously. Woo!! neither core went above 65 degrees.
[Picture above taken right before the CPU powered down. 120°C (248°F) is the hottest I've ever seen a chip get while monitoring it, hopefully there was no permanent damage done.]