I have the same problem. I have put in a new hard drive
because the old one will not boot at all. I use the old
drive as a slave so I can at least access my files. A
fresh factory restore was installed on the new drive.
Within 2 days, again it was rebooting itself. Within 4
days the new drive cannot boot up. Everything is in the
process of be reinstalled, again, by COMPUSA. They claim
that there is no problem. When this problem started it
also included no video on startup, sporadically.
Eventually, the motherboard was replaced. This cured the
video situation.
I'm curious as to what system and processor you have. My
system is a Compaq 8000Z with an AMD Athlon 1900
processor. I will let you know if I have any better luck
when I get my computer back later this week.