I've been through many starters on 123 diesels. The problem you describe could most likely be fixed by simply replacing the solenoid as long as it was not a cheap rebuilt to begin with.

If it WAS a cheap rebuilt, get a Bosch rebuilt or get one from the dealer. The good news is that the starter is a piece of cake to change on the four cylinder. You definitely cannot say that for the 300D, however.

Good luck,