Server rebooting itself

We have a HP Netserver E45 that keeps on rebooting itself. it is connected in UPS along with other servers. I have switched the sockets in UPS. the frequency of rebooting has gone down, but it still reboots about once to everyother day. Other servers are not having any problem.

What should I look to see why this server is rebooting itself. Server is runnig Novell 3.12.

Open to suggestions on what to check