Sporadic lockups with not much to go on
So here's the deal:
My machine (Athlon 64 running Slackware) has been locking up completely (in a "no response from the keyboard, no response to TCP/IP packets, have-to-stick-a-pen-in-it-to-make-it-reboot" kind of way) at seemingly unpredictable times. The lockups may have something to do with periods of high activity -- they seem to happen most often when I'm watching a video, compiling something big, or using the Firefox Flash plugin heavily.
At first I thought the lockups were related to X (I use Xorg). But it happens sometimes at the terminal before I even start X. Then I thought it might be a problem in my kernel, but I have encountered the lockups in both 2.6.9 and 2.4.26. I also checked whether it was a motherboard overheating problem by hooking up my extra fan and running it with the tower open. The lockups persisted.
I can't think of any further way to troubleshoot this. I know this isn't very much to go on, but what would you do next if you were in my shoes?