regular (but not predictable) complete freeze-ups

I'm experiencing regular freeze-ups of my 5.2.1 system.  It is a p4 2.8
running hyperthreaded, 800 FSB, Serial ATA, 8x AGP.  So this is relatively
new hardware which is sometimes buggy all by itself.  I'm running X with
XFCE4, and periodically I experience complete system freeze-ups.  No mouse
movement, no caps-lock light on the keyboard, no ssh from a remote
machine, nothing.  I don't think I can reproduce this problem on demand,
as it doesn't seem to be related to a specific application.  It seems to
be more closely related to system load, although this machine has compiled
for days at a time with no problem.  I am also having problems with
gnucash, and was trying to run gdb on it last night, and did consistently
freeze my system by allowing gdb to get well into its work and then trying
to open another program.  the two that froze the system were aterm and
firefox and they were the only two I tried.  This has happened when I
didn't open any new programs too.  I suspect that pan has caused freeze up
on a few occasions, and mencoder too.  Those freezes did not coincide with
my starting any new programs, but rather happened after they were running
a while.  If i'm only running aterm, firefox, and irssi it will seemingly
run forever without crashing.

All this description and no question yet?  heh.  well my question is how
can I set up an environment that will tell me what is causing these freeze
ups?  do i need a null modem cable on the serial port to a remote
terminal?  (i've heard freebsd makes a last ditch effort to spit errors to
the serial port when it crashes) what might i try?  I see nothing in the
logs.  because this appears to be an instantaneous halt to all operations,
that doesn't surprise me though.  perhaps i should see what my milage is
with another os, and if it remains bad begin to investigate my hardware? 
is there a good hardware burn-in and/or analysis program for freebsd that
i might try?