IBM e326 + RHEL4 => unexpected shutdown

[Patrik Koch <patrik koch philips com>]

I've just installed RHEL4 on an e326 server, with undesired result:
After some time of inactivity (around 2 hours) the system goes down and
halts.
That's a bad behaviour for a server.

Any ideas where to look at? I checked BIOS, crontab, etc.
The hardware is new for me, also RHEL4 (kernel is 2.6.9-5.ELsmp).
/var/log/messages contain lines like:
      Apr 25 17:35:11 ls0150 shutdown: shutting down for system halt
      Apr 25 17:35:11 ls0150 init: Switching to runlevel: 0

Thanks in advance for your help!

Patrik