This is not going to be a great solution or probably the answer you would want to hear either but we'll see if it helps you at all. Awhile ago I had the same problem with my computer, while I was playing certain games the computer would reboot itself. Mine ended up being a conflict with my video card. I'm not certain that the video card was faulty necessarily, but when I used a different video card it stopped doing it. I think it may have been overheating or something.
In my case I actually put an older video card in, which wasn't quite as good, but it stopped rebooting and I put the other video card in a different computer with no trouble (but I didn't use it for games).
Well hope maybe it is somewhat helpful, if it is an option for you maybe you could try it. Wish I could help more.
