I had an issue like yours, any game would shut my PC down, after about half an hour of play  it turned out to be blocked fins below fan on my processor and the seating compound was not doing its job, so I stripped down and cleand every thing and re-pasted and all was well, I would also check your graphics card has nothing restricting air  flow, and you are using two rails from your PSU to power it, however if it's not an hardware issue then check to see if you have the latest BIOS installed for your system, and as BehindYou_Tom said look at your event viewer, to see if that shows any faults recorded, you could simply have a faulting PSU that cannot deliver the correct current to power every thing when playing high end games.