Sorry I haven't replied sooner, but anyway, here goes... 2K used to restart on me quite a bit at random times - usually with a kernal exception. After installing Detonator drivers, the problem got worse and the exceptions were now in the graphics drivers. I eventually came to the conclusion that some devices weren't fully compatible with my motherboard (GA-5AX) and that was the cause. However, after reading an article recently about random errors/reboots being caused by faulty/underpowered PSUs, I've since had no problems (I updraged to a 400w PSU from my old 230w one.).
Hope that helps !