This doesn't happen in all games I play but most recently, Darksiders and Bioshock Infinite have caused my computer to crash after about an hour of playing. I checked and this is not because of overheating so I don't know the cause.
Also to turn it back on, I have to unplug the power cord from both the outlet and the power supply and then plug it back in. If I try simply pressing the power button it is unresponsive.
Has anyone else experience this? As l said this doesn't happen in all games, but it seems to be only the first time I launch them.
Both the motherboard and the PSU have safety features that can cause the computer to shut down, but I agree with Brett928s2. It sounds more like a PSU problem.
Is your computer plugged into an older UPS (battery backup)? Some older battery backups can't deal with the active PFC (power factor correction) on newer PSU's and will cause symtoms like this. This is assuming your power suply has active PFC. If this is the case, try plugging the computer directly into the wall and see if it fixes the problem.
I am also having this same problem. Right now I play Bioshock Infinite and Dishonored. I have an Asus G73JH and it's always plugged into the wall. My bios and graphics drivers are up to date. Even sometimes if I leave it sitting in a game menu it will simply crash.