My 300SE had that prob and it turned out to be both the window regulator teeth and the sheet metal onto which the regulator bolts onto was bending. The sheet metal fatigue seems to be common on the Se's.

Same car's rear driver side window would not lower with motor, That turned out to be just the plastic bushing referred to by the previous poster.
Anyway, symptoms have diff incarnations.
Take the panel off and snoop.

Den

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1979 300D 220 K miles
1995 C280 109 K miles
1992 Cadillac Eldorado Touring Coupe 57K miles SOLD
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1979 240D 140Kmiles (bought for parents) *SOLD.
SAN FRANCISCO/(*San Diego)
1989 300SE 148 K miles *SOLD