Random reboots on 4.99.72

[Paul Goyette <paul%whooppee.com@localhost>]

I've just updated my systems from 4.99.67 to 4.99.72 (sources current as 
of about 36 hours ago) and so far I've had at least half a dozen random 
reboots.  In all cases there are no crash dumps.  (Well, that's not 
entirely true:  once I caught it dumping to disk, but savecore didn't 
find anything on the reboot.)
This has happened at least four or five times on a very lightly-loaded 
dual-CPU amd64 box.  This box does nothing except NAT and routing 
between the internal and external network.  It has also happened just 
now for the second time on a fairly heavily loaded quad-CPU amd64 box, 
which is my NFS server and build machine.
I don't have serial console, and all boxes generally run X, so it's not 
very likely that I'll actually catch it in the act.
Is anyone else seeing anything strange with -current?  Any hints on how 
to debug this?
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