Any way to debug a hard lockup?

For the last few days, my amd64 box (Athlon 64 3200+, 5.2-CURRENT,
updating just about daily lately) has been locking up hard fairly
often, and I have no clue as to why.

Checking the logs after rebooting reveals nothing at all out of the
ordinary.  The box just "seizes up" and that's that.  Only thing to do
is a power-down and restart.

Changing from SCHED_ULE to SCHED_4BSD appears to make no difference at
all.

My other box, an older (32-bit) Athlon, has been rock stable.  I've
been keeping both machines updated simultaneously to exactly the same
revisions, using as near-identical system settings as possible also,
the main difference being I'm running X.org/GNOME on the amd64 box and
no X on the i386 box.

Any suggestions?  Instinct tells me it may be a problem with some of
the ports I'm running (X.org/GNOME, hence the Cc:s), but how to verify?
Sure, I could run without X a while and see how it goes, but that still
won't help to isolate the problem, if it is, in fact, port-related.

Thanks.

-- 
Conrad J. Sabatier <conrads_at_cox.net> -- "In Unix veritas"