Diagnosing periodic reboot?

[David Kurtz]

My hobby FreeBSD box has started to reboot at semi-regular intervals,  
and I can't figure out why.

There seems to be no warning, no message at the console and no  
indication of (or, probably, I don't recognize) the problem at reboot.

This happened a few months ago, and for some reason I deduced that  
the computer was getting too hot. I installed a better fan, and that  
seemed to do it.

Now it's happening again, and I'm not sure I can blame heat.

What log files should I be looking at?

(BTW, hardware is a Shuttle SB51G enclosure with a 2GHz Intel  
Celeron, circa Jan. 2003)