No, it occours in the times in my original post

. The tab thing just happened to me again a minute ago. I wasn't alt+tabbing out, I was just holding down tab to check the scoreboard to see what team to join, and I clicked a team while I had tab held down. It instantaneously restarted my computer. No message, nothing. I held tab, clicked, screen went instantly black and rebooted. I can duplicate constantly, like I said it happens about 50 to 75% of the time, I'm just tired and forgot to let go of tab this time.
The other instance was strange in the sence that I hit T to bring up chat window while running, then alt-tabbed just to check my msn quick while I was approaching something and it instantly rebooted there too.
I thought it was an overheat or something like that too, but the only time my computer restarts is in those scenarios. And again, I run the game smoothly on low settings. I very rarely experience choppiness or anything even, it seems to run the game fine, and I've never (even after marathon single player sessions) had my computer restart, or even had it lock up or crash on me. The worst I ever have is when new enemies/reinforcements come in it lags for like 1-2 seconds as they all pop on the screen, that's it. And again, I can duplicate these 2 bugs pretty much at will, same circumstances and all!