How about a terminal program (tip, putty, minicom, telix, JrComm, etc..)
with a massive scroll back buffer logging to disk and let it run?
Have you tried provoking it with heavy network, or memory usage, or disk
activity?
I have an IPX with a weitek powerUp! cpu + rom that was aaround 2003
getting a bit on the unstable side with similar lock-ups. I ended up
de-soldering and re-capping the entire board + PSU, and thus far touch
wood it's not sneezed.
Another thing that may be worth thinking about is an expect script or
perl script driving the serial device that'll send escape sequences
through?