When your system just randomly shuts off that 95% off the time has to do with overheating CPU, GPU, PSU, or just some power failure.

When you play games you use almost all your systems resources. Ram, GPU, CPU, and PSU. If your system doesnt have enough power to run all these it can cause it to shut down. If your CPU is over heating it can cause your system to shut down. If your GPU is overheating it can cause your system to shut down.

Sense you claim its not overheating. (Not knowing how you came to this concussion I would not rule it out as the culprit.) Than it is most likely an issue with your PSU not having enough power to run all your stuff.

The easiest way to solve a lack of power issue is to underclock your GPU. When playing games higher end GPU's suck up tons of power. Even on PSU's that claim they can handle said card it can still be an issue. I would keep underclocking the GPU by small amounts untill the problem seems to stop. This will also help solve the problem if it is an overheating GPU issue as well.