Check the CPU temperature anyway. If it's below 60c under full load, you should be fine. If the restart is happening only when playing that game, but not while idling, typing text or browsing Wikipedia (these 3 are not CPU intensive), then overheating is likely the culprit - try some other CPU-intensive process like compressing a few gigs worth of files or doing video processing (especially with the H.264 codec). If you don't get the restart while doing this, then it could be a stop error or blue screen of death. In this case, you may have a driver-related problem. This is otherwise about as far as I can go.