At least on coupe W123 cars have a 90 degree piece of metal that has a rubber coating on it where the regulator hits.

I’ve had a car where the piece, which is bolted into place but can be adjusted, flipped the wrong way, yet had enough adjustment that it almost stopped in the right place but was a bit off.

I have to wonder if a stop is not correctly adjusted and the rubber piece is grabbing and then the torque is making the mechanism jump.

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)