Problem when gaming on my PC

I've just installed Wolfenstein on my PC with Windows 7 Pro and I am overclocking my graphics card with EVGA Precision. When I start the game it runs fine, but after a minute or so the game will start to lag, I have to reboot the computer to fix the problem but then it will happen again. The only way around this is to not overclock the graphics card at all, which is fine as the game doesn't lag at all, I'm just curious as to why it does this on some games... I can overclock the card and play other games, albeit one other one, can anyone shed some light on the problem at all? It's not as though the card is cooking up (averaging 65C whilst playing games) and the settings aren't uber high...