I have had clients who experienced such phenomenons (LOL) with their computers. There were 2 recent cases where the desktop machines reboot a few mins after they are up and running. You could also take to account there two possibilities :

1. Motherboard chipset drivers are either not installed or not updated.
2. Cooling systems are not enough or processors are defective

With the #2, usually we take the computer for a break-in and leave it on for a few days to test if it is working properly.