On port-alpha a second report has been made where the root cause was a
misaligned access in sbdrop(). I've opened kern/8673 for this.

I'm not sure how to find such an intermittent bug. Unless someone thinks
they know of a recent blameworthy network mod, about all I can think of
is instrumenting sbdrop() and the zillion macros it calls to panic on
misalignment, this way the i386 -current users could help locate it. :-)