[CentOS] Server hangs on CentOS 5.5

> Video is on the MB.  It doesn't seem likely that it's
> the video, since the system doesn't respond to network
> when it crashes.

bad video hardware or drivers can easily crash the system

If its running an X windows display of any sort, I'd suggest trying it 
in text-only mode.   in /etc/inittab, set the default runlevel to 3 
instead of 5.   this leaves the video in plain VGA text mode which is 
far less likely to crash the system.

     id:3:initdefault:

bonus, if this is a server, and thats a shared memory video system, 
disabling the graphic modes reduces the memory bus contention, speeding 
up the whole system by some percentage.