1983 633Csi Clutch Problem

It's a 1983 633 Csi with 220000 miles on the original engine. The clutch pedal was spongy and I replaced the master and the slave cylinders then bled the
system. For about 6 months I didn't have any problem then the clutch pedal started to stay
on the floor and didn't come back. In order to use it again I had to pull the pedal up and
wait about 5 - 10 minutes. When the problem showed up more frequently I thought that the
parts were defective and replaced both of them one more time. These new parts acted fine for
about 4 months and the problem appeared again. I took the car to a mechanic. They checked the parts and found nothing wrong with the clutch
hydraulic system and offered to bleed the system that didn't change the result.
I wonder if anyone had such a problem in the past and how solved it. Or does anyone has any thought about it.
Only two other things that I can think of are: a) The clutch system (the pressure plate) has a problem and moving somehow. By the way I
have 125000 miles on the clutch but it's not slipping at all.
b) The crank is moving back and forth moving the clutch assembly together. This is a
known problem with some Mitsubishis but not with BMWs.
I would appreciate anyones opinion.
Thanks Phil
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