debugging lockups

[Vladimir V Egorin]

I have a FreeBSD-current machine with a system built on 
Feb 26, 2004.   The system hangs hard at seemingly
random times.    It had the same problem in 5.2 and
5.2.1 releases (but not in 4.8-STABLE)
I have tried to investigate this further, as described
in the FAQ.  I have 
options  	DDB
options         BREAK_TO_DEBUGGER 
set in the kernel config; the problem is that the system
doesn't react to keyboard input (ctrl-alt-esc) after
the lockup.  X is not running.
I can reliably reproduce the problem by starting a process that heavily
uses disk and memory (e.g. maple computations, or mencoder), and then
killing it.  The system locks instantly.

I don't believe it's a problem with the hardware at this point:
I have tried replacing memory in the machine with no luck,
and the system is very stable under heavy loads in 4.8-STABLE.

Any suggestions on how to proceed?  Do you think I would have
more luck with a serial console?
Thanks,
-- 
Vladimir