I don't know if you fixed the problem, but the plastic clip running up and
down the track was the problem for me in my '84 300SD. I replaced it
using a new one ($5 part) and it worked fine. I had to do my own
engineering when it came to putting it together, so I used a self-guiding
screw and superglue to put it together on the metal arm. Hope this helps,
my window works fine now. Took about an hour to undo/re-do everything.