FC4 install rebooting at night for no reason

[Mark]

Hello,

I have a newly installed FC4 machine that seems to reboot at night for no apparent reason.
I checked /var/log/messages and the log contains all messages from the startup, but nothing that looks like a reason for a reboot.
I ran chkrootkit and rkhunter and found nothing.

The machine does not have a lot to do, it is mainly used as a backup server that other machines mount via NFS to store nightly
backups (tgz files) on.
It is also used as a firewall for the secondary internet connection and as cfengine server.

The machine is running in text mode only.

The machine comes back automatically, the only reason I noticed the reboot at all was that I called "uptime" and the machine claims
being up for a few hours while it should really have been a few days...

Does anybody know what could cause a reboot on the software/OS side of things?

Thanks,

MARK