Computer cuts off during some games

I have been able to play WoW at max settings (shadows, multi-sampling, particles, etc..), MW2 at max, spore at max, minecraft at max, and starcraft II at ...high... with 50-60 fps (monitor's highest setting) and no issues, but today I tried to play Age of Empires Online, and after an hour, my computer cut off and restarted. I changed some settings in the game and tried again and after 10 minutes, it reset once more. The third time, I set my cpu fan to manual-maximum ( thinking the intel safety functions were cutting it off at a certain heat ), and it cut off after 10 minutes again. The time intervals are a rough estimate, of course.

this problem only occurs on windows 7 (i haven't used anything else for long enough time to recreate the outcome). CPU temp doesn't exceed 55C on anything i've done (heavy gaming). Case is open currently and has 2 fans and several ventilation openings all over it. Graphics card is hot to the touch, but I cant think of any way to cool it better....

My specs are :
Gigabyte GA-H55M-USB3 rev 2.0 bios-F11
Intel Core i5-655k 3.2gh (not overclocked currently)
( 2 physical cores, 4 virtual,
core 1 temp : roughly 26C
core 2 temp : roughly 32C
at idle )
Cooler Master Hyper N520 CPU Cooler
Diablotek PSDA600 DA Series 600W Power Supply
8gb (2x4gb at 1333)
Radeon HD 6950 2gb (flashed to 6970)

OS: Windows 7, Ubuntu Linux 10.10, OSX Snow Leopard 10.6.6
win7/unbuntu on HDD 1, OSX on HDD2.

One Final Thought: I've heard Wow and minecraft (the two games I use most) are CPU-dependent, and not so much GPU dependant. Could it be that Age of Empires Online is Graphic card-optimized, putting more stress on the GC and it's overheating causing the mobo to cut off?
 
UPDATE:

problem still persists, but I have replicated the results twice while watching temperatures. card temperature never exceeds 68C; cpu core 1 stays around 30C and core 2 stays around 40C. I dont think it's an overheating issue.
 
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