Frequent Shut down of Server HP DL 380 G5 server

I have an ISA Server 2003 running on a HP DL 380 G5 server. The server recently started shutting down in 2hours Intervals after prompting me that “The server will shut down in 30 miniutes”. I have to all of the time go and restart the server.

What do you think could be responsible for this frequent shut downs?

Thanks