Is it regularly at the same time every day? Or is it every hour/whatever or is just random? Have you got any software installed that automatically turns it off? Not like a virus I mean, just a utility. Have a browse through some programs. That is if it's at a regular time every day. Unlikely, I know. If it's at random times, it might just be the power (not the PSU, the actual plug). Unplug the computer, then plug into a different socket. Perhaps also unplug the motherboard power supply and then plug it in again, it may be a loose connection. Have you noticed other things turning off at the same time? That will help determine whether its hardware or software, or a power supply (mains) issue.