Having read everything else suggested and all you've tried, one last
thing occurs to me.

The E28 is subject to cracking pedal brackets.  The stamped metal
assembly that holds your master cylinder and the clutch pedal cracks
from stress and wreaks all kinds of havoc because of it.  It can
destroy a new master cylinder quite quickly, so check yours as you do
this job.  You can either replace it with a new one (they aren't
expensive) or weld your old one up (with reinforcement in the obvious
area where it cracked).  Good luck.
--
C.R. Krieger
Been there; haven't cracked mine yet ...

1986 BMW 535i.  I bought it a few years ago with a bad transmission and only
120k miles, and a friend of mine dropped an '83 junkyard transmission into it
and a new clutch.  It's been running well, and I have slowly been cleaning
various things on it up.

Starting around 220,000 miles, the clutch began sticking closed.  That is,
I'd put my foot down, the pedal would stick to the ground, and the clutch
would remain engaged.  I could pull the pedal up and pump it down a couple
times and the clutch would open properly again.  It started happening now
and then, and then more and more often.

I replaced the plunger on the clutch master cylinder, which was leaking,
on the suspicion the seal in it was bad.  Didn't fix anything.  Took it
to a respected BMW indy mechanic around here, who swore I did it wrong and
replaced the plunger again.  Didn't fix anything.  Replaced the whole master
cylinder.  Didn't do anything.  Changed the fluid and replaced the slave
cylinder.  Still no go.  

Now, I am assuming that there is no possibility of the line between the
two cylinders being an issue, and no backflow valve anywhere in the system.
Is this correct?

Recently it got worse, and I took it in to him again.  At this point, we
figure it has to be something inside the transmission.  He drops it, and
replaces everything that looks even a bit suspicious, namely the throwout
bearing release lever, the lever pin, the clutch release bearing, and
the shift bushings.  Everything else looks clean, though neither one of us
could see how any one of these would cause the problem.

Putting the thing back together, the problem is now actually worse, since
the friction point is closer to the floor than it was before.  The car is
now doing it almost all the time, making it impossible to drive except on
the highway.

Help!  Anybody have any suggestions about what could be causing this issue?
--scott