If this is only happening during gaming, I'd suggest that it may be your GPU. Occasionally, pre-OC GPUs from 3rd party companies can be faulty or badly optimized compared to their stock cousins. I know for a fact that many Gigabyte 560 Super OC users are reporting that their cards are causing problems, so hopefully it's a similar manufacturer issue.
Equally, your 12V rail may be damaged or not connected properly. Check the connections and test the rail for issues. Connect another GPU to your PC and see what happens under the same conditions. If it still crashes, this is likely an issue with the 12V.
Best of luck