Happened sort of like that on my 1979 245DL (RIP!). The clutch would not stay in adjustment. Every time I adjusted the freeplay, in a few uses the freeplay was gone and the clutch would not disengage.

Looked in the book. There is an arm across the clutch. One end you see, where the cable attaches. The other end is at a ball joint or pivot on the opposite side. The mainshaft fits through a hole in the center section of the arm, and that's where the throwout bearing is mounted.

Step on the clutch, the arm is pulled forward, the throwout bearing engages the clutch release parts, and it disengages. On my 1979, that arm was slowly bending...and bending...and bending, until things were almost locked up.

Took in to my mechanic, had the trannsy dropped, clutch renewed, and a new arm installed.

Whatever is going on in your tranny, that arm is likely at fault. It will require dropping th tranny and getting into the clutch stuff.

Good Luck,

Bob

:>)