F11 just dies; how to debug?

[sean darcy]

I'm using F11 on a server that among other things runs asterisk as our 
phone system. So it's really important it runs.

But recently, about once a month, the server just goes down, powers off. 
It's easy to start - just push the power button. But why does it go off? 
Nothing in syslog.

Bios is set to reboot on power loss. I've tested that by pulling the 
plug while it's running. When I put the plug back in, it powers up and 
boots like it should.

Any help appreciated.

sean