The symptoms are a simple case of overclocking instability. Many processors will cause the system to shut down if clocked too high for stability. Mine does it, my brothers does it, and basically everyone that has tried OC'ing has done it. You have to find a stable clock speed before it starts shutting down under load. The shutting down is caused by one or many of a large number of things, ranging from the chipsets being too hot, to the PSU not applying enough voltage, and even to your CPU simply not being capable of stability at the OC'ed speed.
I really don't know what to tell you besides to clock your CPU back down. It is sort of odd that it doesn't do it in games though...
This very problem is why I choose not to go with OC'ing. It's much less annoying to simply buy a better processor.
*EDIT* May I ask why you're even OCing? Your CPU is more than enough for any game on the market. You won't notice a difference in any application when OCing that CPU.
-Lasereth