Again make sure the alternator is putting out enough voltage to charge the battery fully. Many old mercedes have sub standard voltage outputs from their alternators. If you have the six cylinder 603 engine they can be a little cold blooded when starting. You did not post what you have I believe. Our moderator recommends two glow plug cycles for those engines.

If you have the five or four cylinder 616 or 617 engine check your valve clearances if they have not been done as well. This is almost a mandatory thing to do every fifteen thousand miles. Many engines out there cannot remember the last time they were done. This can effect start ups.