Starter spins just after cranking

Hi All,
My '83 300TD starter will occasionally make a momentary grinding sound and then spin until I turn the key away from start position at which point it continues spinning down for a few seconds and then abruptly stops. Retrying a moment later it cranks normally and the engine starts up no problem. The chance of spinning seems random, it happens instantly when trying to start or after a few cranks of the engine. Any thoughts on what to look for?
thanks,
-v