Computer crashing on only some games

I'm at my wits end. My computer is is fully shutting down with no warning during some games. Simple games Like Dota 2 or bastion I have no trouble playing(most of the time). While games like Planetside 2 or Skyrim crash within 5-10minutes. (interesting side note on Skyrim is the more mods I installed, the faster it crashed).

My computer.
- i5 2500K quad core sandy bridge
- 8Gb DDR3 ram
- XFX AMD HD 7970 (1GB ram)
- 750Watt power source

To get some questions out of the way. Yes I've checked all my temps, never over 50-60C on my GPU and CPU and . I even went so far as to replace the thermal paste on my CPU. I have the above the recommended hardware for the games.

Any help what so ever will be greatly appreciated.
 
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