Also, please list your full system specs including detailed information on your PSU and any overclocks.
Use Coretemp and GPUz to find your idle and max load temps for both the GPU and CPU then post them here.
It sounds like your CPU is overheating, most "modern" processors will shutdown at a certain temperature. Reason it doesn't happen while not playing is mainly due to "Cool 'n' Quiet" but also the CPU isn't loaded. You may need to make sure the air is flowing okay in your case, dust is removed from your heatsink and fan. If that doesn't work, you may need to replace the heatsink and fan.