System hangs after a while

[Steve Loftis]

I noticed lately that my system just freezes and becomes unresponsive 
after several hours. It doesn't matter whether I'm in X or if i'm simply 
in the console, but after a while the screen goes blank, and my input 
devices such as keyboard and mouse simply don't work.

After a restart, the keyboard and mouse still don't work and I'm stuck 
at the bootloader. Therefore, I turn the system off, unplug both mouse 
and keyboard and turn it back on. It works just fine... until the 
initial problem starts over again.

What could be the source of this? I am running FreeBSD 5.1 and I'm 
running an edited kernel simply to add sound support.

Any suggestions?