Don't just apply MORE Gleitpaste (Mercedes Sunroof Grease) ,first clean the rails.
lubricant for sliding room w123
AND as the other posters have cautioned,Be Very,Very,Very careful with
tightening the "Clutch" mechanism in the Trunk...You can create more problems than you THINK you're fixing.
Your problem MAY be part # 8 in the Cyrillic pictogram below:
[The reason I'm showing you #8 is because it "Takes the most Strain" in the
Lifting operation...AND if the S'Roof has not been maintained (Cleaned and
lubed at each oil change) .It MAY suffer a breakage.]
[The only other thing I can think of is that somehow one side of your S'Roof
has gotten out of adjustment as far as the lifting operation.][Not Likely]
[Do Not go "PLundering" around in your Sunroof's Innards until you know what
you're doing...The Dealerships charge $1,000.00 just to change a Sunroof fuse.
(If they can get away with it.) ]
http://www.detali.ru/cat/oem_mb2.asp?TP=1&F=126035_568&M=116.965&GA=722.324355&L=765.706&CT=F&cat=568&SID=78&SGR=060&SGN=02
(Yeah,It's from a 420SEL ,But your S'Roof is the same)