Hardware Computer crashing on only some games

MarniMo

TS Rookie
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.