I had the same problem. I pulled the starter and ended up finding all the teeth on both flywheel and starter good. I was worried I lost some teeth on the flywheel. After changing the starter, the problem is gone. I would pull the starter and have it tested. I tried changing my solenoid and the new one didn't work so that might be an option if you find the right one.

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85 300D Turbo

I have not pulled a Mercedes starter, but most starters you can oil the shaft once it is out and on the bench. There is a specail lube available. Cushmans needed to be pulled out about once a year and lubed.

Man, if I'm going to pull the starter out, I'm going to replacement with something I know is good. Never had problems with a Bosch reman.

I've gotten pretty good at changing these things over the years, but it's still not something I look forward to. Adjusting the valve clearance, and even changing the oil, is a labor-of-love. R+R the starter on a five-cylinder in a W123 just plain sucks.

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er