As this is RHEL you can run sos
and raise that with RHEL support.
What is the poweroff
signal? IPMI? Have you got this:
yum install OpenIPMI OpenIPMI-tools chkconfig ipmi on service ipmi start
There is a chance that IPMI have some bugs or you are sending a wrong signal. See if you can update some things as you should not be running RHEL 5.0 any more due to millions of critical patches to it!