[7.0] /bin/sh segfaulting during boot

[walt]

I'm now running RELENG_7 on three x86 machines, but (only) one of
them is behaving badly during bootup.

Once the rc.d scripts start to run, I see /bin/sh crashing dozens of
times before I finally see the login prompt.  A few of the normal
processes actually start okay, like ppp and named (this is my home
firewall), but many normal daemons like syslogd and sendmail are
not running.

The backtrace for sh.core isn't very helpful because there were no
debugging symbols (but I'm recompiling now).

I found an old statically-linked sh from 2001 still on the machine,
so I substituted it for the current /bin/sh and everything works
perfectly.  Even the current sh which crashes during boot seems to
work fine after I log in -- I can use it to run the rc.d scripts
manually with no problems.

Any ideas why /bin/sh would crash when being invoked by init, but
works fine otherwise?

This particular machine is old -- a Cyrix (i486) cpu and only 64MB
of RAM -- but it's still stable as a rock.  I've already rebuilt
world and kernel twice with no problems at all (which took almost
a week).

Any diagnostic experiments I could try?