I had the same problem with the sunroof in my 1994 Euro C180 last year. There were 2 things done. First I had the rails greased by a Benz tech using the genuine Benz grease. Then I changed to a new heavy duty battery and wow, it works like a dream now. Previously it was struggling to move forwards and backwards. Worse, it refused to tillt up , so I had to give it a shove from below to give it just enough pressure to rise. The worst thing was that a few techs quoted me A$2000 to take out the whole thing, replace all the broken or have to be replaced parts, grease it, put it back in plus all the work needed to do the roof lining work required . The grease and the labour only cost a few dollars instead.