Had same problem with my '86 300E, I lubricated the rails but problem remained. Ended up to be the cable from the sunroof to motor was binding inside the guide tube. In my case the cable attaches to the sunroof and runs through a tube back to the motor in the trunk. Some of the little white plastic ferrules on the cable were worn and binding inside the tube. I replaced the cable which resolved the problem. The fix was not that bad, I followed instructions from the MB cd, worked carefully, and it took about three hours.