I had a 83 300SD that did the same thing, the gear that pushes and pulls on the toothed nylon piece had worn material off the teeth at the top and the bottom. The gear would slip on the nylon when almost fully retracted or raised and the motor did not know when to stop. I had to buy a whole new unit. Now I see parts to replace just the mast and the nylon drive piece.

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1987 300D 146K Original #14 Head
One eye on the road, the other on the temp. gauge

Gone But Not Forgotten
1983 380 SEL Long gone
1983 300SD Wish I hadn't traded it in on a 90' Corvette
1989 300SE My all time favorite
1995 E420 Went like a bullet