System reboots itself

From Mac OS X Server FAQ

Mac OS X Server has a service called watchdog that is responsible for restarting important processes if they die, and also for rebooting the machine if it detects a freeze. Basically, when the watchdog process fails to reset a reboot timer in the power management unit, the PMU assumes that watchdog and the rest of the system have frozen, and triggers a hard reboot. Open a terminal and type man watchdog for more information. Moreover you can speed up mac os x or any other with mac how to