Diagnosing reboot under load

[Micah]

My desktop system just started doing this last night.  I was upgrading 
Gnome using the handy shell script they provide.  It looks like sometime 
around 11:30pm the computer reset.  This morning I'm trying to reinstall 
all the software that got lost in last night's reset and I get another 
reset in the middle of compiling.  The last message in /var/log/messages 
before reboot is:
Nov  6 10:41:08 trisha ntpd[489]: kernel time sync enabled 6001
Nov  6 10:58:14 trisha ntpd[489]: kernel time sync enabled 2001
Nov  6 13:02:57 trisha syslogd: kernel boot file is /boot/kernel/kernel

I just ran memtest86+ and there's no memory errors.  I'm guessing it's a 
hardware issue, but how do I diagnose it?

Thanks,
Micah