I've got a weird issue with one of my test servers. It has happened twice now, originally, I thought it was related to my UPS battery going bad (and it probably was then), but last night, completely unplugged from the UPS (as I haven't ordered a new battery yet), by test server went down around 1:00 AM.

So far, I have no idea why. I've checked the syslog and the kern.log (along with apache logs), but nothing is indicating a direct failure that would cause a system halt.

I've just installed munin and munin-node to create system graphs (described here), what else should I be checking? The CPU and MB temperatures look good on reboot, and I've never seen them get high temps even under major load.