I'm almost positive it's your power supply.I just took a system back for the same behaviour.This was on an install of XP Home that all of a sudden was missing the MBR intermittently.I tried repairing with the cd and it kaept rebooting.I couldn't get the recovery patition to function either.Couldn't get Dos boot disk to function with fdisk.Changed vid card,HDD,cables,memory,disconnected FDD and cdroms.Then I changed the Power supply and everything worked fine.