This statement is true, if the Power Supply isn't defective in some way. It might be worth using a multi-meter (or other means) to monitor the 12v and 5v rails while it is under load. Might be when idle the supply is fine, but under heavy load one of the rails is dropping enough to trigger the supply to shut down.
Are you sure its not the "over temperature" after all? Could be the settings are set too low in BIOS?
I dont know what OS you use, but windows have an event viewer, that might give you some clue...if it manaages to log it before shutting off that is. For XP: windows-help-central.com - windows help central Resources and Information. I'm sure I wrote something else in the run box, I just dont remember what atm, its been ages since I used that.
That's pretty hot, take the cover off and see if your heat-sink and fan aren't covered with dust, as that should be the probable cause of the overheating.
minding my own, playing GTA 4 and it shuts off, temp at 84c (personally anything over 70 is pushing it)
grumble I dont feel like dealing with this right now. I just cleaned it out and replaced heatsink goo a week ago, was running fine at 52 under full load