Hello! Going through my new 1980 300CD. I looks like someone tried to put in a manual crank regulator out of a 4 door, so i removed it and found a proper power one.

I tested it out of the car - strong, gears are all good, smooth, etc. I lube everything and put it in, get everything adjusted, but it's doing something strange. When the window is 80% up, it will clunk, the window will fall 3 inches, then continue up and repeat. If I help it past that little bit, it's fine. There isn't any resistance I can detect right then, unless its in the rubber channels...

Any ideas?

Thanks-
-rj