sudden crashes

[dbclinton]

Hi,
I recently installed Edubuntu 10.10 on an old Compaq Evo Desktop to
serve a computer lab of five clients (old Compaq Presarios). The network
is running, but once or twice a day the server will crash (blank screen,
no keyboard or mouse). It doesn't seem to matter whether the server
itself is idle or busy or whether there are clients logged in. I
couldn't find any warnings or indicators in the system logs so I assumed
that it was probably a hardware problem. Memtest (via the GRUB menu)
came up fine so that rules out the most obvious issue.
I'm not actually in the lab all that much so I rely on reports from
students. Just today I was told that even when the server crashes the
clients keep running. And that they "thought" that the server might have
come back to life on its own after an hour or so.
Any ideas what this could be and what can be done to find out more?
Thanks,
David Clinton