Sounds like a cable problem.

There is a clutch, keeps the motor from burning up or destroying the gears. It is adjusted in the trunk by pulling the 17mm plastic nut off, loosen the locknut, and tighten or loosen the center stub to tighten or loosen the clutch respectively, then tighten the locknut.

More likely it is the cable grease being gummy and causing too much resistance to the motor. In this case, you need to remove the cable, clean the cable and as much housing as you can, and re-install the cable.

Removing the cable is relatively easy, but can be messy to the interior if you aren't prepared to keep it wrapped and send it out the window. It is attached to the sunroof transport bridge just behind the pop-up vent in the headliner by two 10mm-hex bolts, can be detached carefully through that opening and run out into the cabin of the car. Be careful to pad and protect the headliner opening with this technique, you won't want to damage your headliner!! The FSM procedure involves running the sunroof open far enough to un-snap the 6-8 (can't remember the number) "christmas-tree" clips and pull the front of the healiner piece down/free of the sunroof, slide it back into the roof, close the sunroof, the cable end is exposed. This works fine if the sunroof is operating, ...
Once the cable runs out far enough to disengage from the drive, you simply pull it out into the cabin. Installation is reverse of this, and the drive will need to be "synced" as you mentioned.

Proper M-B sunroof cable grease is available from Phil by clicking the "buy parts" button above. I've cheated and used plumber's grease from the hardware store; pure silicone grease. Worked fine for me in hot and cold, your choice on that one.

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Gone to the dark side

- Jeff