I got this snazzy used Supermicro server, and after I set it up to run SSH, and I started to configure it, it started to reboot every five minutes. I thought I messed up the install, or maybe Xen was causing it. After reinstalling, it stabilized. So I got back to the desktop computer (which is now far away from the server, which has loud fans that sound like an airplane hangar), and the rebooting starts again.
It appears that the problem is a BIOS feature called "Watchdog". When I'm at the keys, the timer's getting reset. When I'm away from the keys, and using an SSH shell, the watchdog reboots the computer.
For now, it's off. Eventually, it may be re enabled, when the system is finished.