When playing games, Minecraft, Left 4 Dead 2, TF2 etc. My computer will completely lock up, the sound will skip for a bit and the monitors will go black. The fans are still spinning and the power is still on but the computer is completely unresponsive. I end up having to hold down the power button and booting back up. It seems to only happen whilst playing games and at totally random intervals. It doesn't appear to be a heating issue.
This is on a custom built Windows 7 Ultimate x64 machine with an AMD Phenom II X4 955 running @ 3.2 with 4 GB ram on a MA790XT-UD4P motherboard with a Geforce GTX 285.