|It sounds as though the CPU is over heating and the safety features in place are causing the automatic shut down. The immediate shutdown after the reboot would most likely be because the CPU has not yet had a chance to cool. |
You might try removing the heat sink, scraping off the manufacturers heat transfer compound, putting on some fresh, and reseating the sink.
If it is possible, you need to find in the BIOS where it is displaying your CPU temperature and monitor that to see if this is the problem.