I had a similar experience. It was the ring gear. The ring gear always stops in a few favored positions, so the starter always engages into these same few spots (the crank always stops on a compression stroke, so, depending on how many cylinders, you get a couple of of "favored" locations). As a result there is higher wear in these spots, and, eventually the wear can cause the starter to lose its abilty to transmit torque to the ring gear as teeth break off. Once all the teeth in each spot are broken off, the starter won't do anything. I had a manual 240D and I could put it in gear and force the engine to turn past these favored spots and get the car to start, but my wife didn't like this ritual. So, I had the ring gear replaced. You have to drop the transmission and remove the clutch on a manual, to get to the ring gear. It is press fit on the flywheel. I changed the clutch wear parts (throw out bearing and clutch plate and pressur plate) while there was, literally, free access even though the clutch was still healthy at the time (about 175,000 miles).

So, I think you have a ring gear failure. Good luck, and I hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)