Login process disappears spontaneously

[David E. Miran]

The login process keeps disappearing.
We are running 4.2BSD on a VAX-11/750 with DMF multiplexors.
Sometimes a line which has had a valid login process will just lose it.
The old process exits and no new one is created.  This happens occasionally
on a hardwired line, but it seems to happen very frequently on our dial-in
lines, especially since we switched to triple speed (2400/1200/300) modems.
It happens more often on our emulex DMF line than on the genuine DEC DMF line.
The only cure right now seems to be the following sequence:
1.  Edit /etc/ttys so that there should NOT be a login process.
2.  Send a hangup signal to init.
3.  Edit /etc/ttys so that there SHOULD be a login process.
4.  Send a hangup signal to init.
This is a nuisance, especially when one line is dead and the other is
busy so I can't sign in to fix the dead line.
HELP
Is this a known problem?  Is the bug in init, getty, the tty handler,
the dmf driver, elsewhere in the kernel.
Any hints or ideas would be welcome.  Actual code to fix the problem
would be even better.
Do other sites see this kind of problem?  Is this unique to dmf lines.
I seem to remember similar problems on our PDP-11 running V6 on DH lines.
P.S.  Our dmf driver is from Berkeley, with local enhancements
if it matters.
-- 
David E. Miran         ...!{seismo,harvard,topaz,ihnp4}!uwvax!uwslh!dem
Wisconsin State Hygiene Lab          (608) 262-0019
University of Wisconsin
465 Henry Mall
Madison, WI  53706