My sunroof moved very slowly when closing when I first got my car. I greased the rails, and it has been fine ever since.
I'm not sure what you can do to fix it, but is there a chance something is caught in the rail?
Maybe it needs to be greased a bit?
Maybe its a gear that is is slipping in the sunroof motor?
I'm no tech, Just brainstorming on things that could be a culprit. I'm a good troubleshooter when it comes to mechanical things, and I try my best to come up with things that might not be obvious, or that might seem way out there, I usually find that trying things that are way out there, can sometimes produce the results needed to fix the problem.
Good luck on your search for a repair.
Personally I would have a sunroof shop take a look at it. Have you looked up ASC? They do sunroofs, I'm not sure if there is one where you are, but I know there is one in los angeles somewhere.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.