There are numerous reasons why an XServe might shut itself down, beyond just the power supply.
As a server, the XServe is designed to run unattended 24/7 and if the system detects a failure it may automatically reboot in an attempt to address the fault.
It may shut down due to temperature issues (too hot), or software issues (critical services crashing or hanging), or the infamous -93 watchdog issue that plagued 10.4.x
You need to check the server logs to see what messages are logged about the shutdown. In my experience (and I've run lots of XServes), the power supply is usually the last of the causes. I think I've replaced one PSU in total.
That said, if it is the PSU, your option is pretty much to find a replacement. They're not considered user-serviceable but are field-replaceable (just pull the old one out and drop a new one in).
Chances are it's a software issue, though.