This is the classic BOSCH solenoid failure .

When it stops fully, you can often make it start by using an unbent paper clip in the terminal box on the inner fender but the smart money is to take the starter off (yes, I know, I do it on hot engines, no complaining) and take it apart, clean and lubricate it , add new brushes (maybe, eyeball them) and install a brandy new BOSCH or (again, -maybe-) Echlin brand solenoid and forget about it for another twenty years .

I have had short lived success with any other brands so beware .

No need to put out big $ for a rebuilt starter unless it doesn't crank with authority, then it's still a maybe, take it to bits and look at it .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better