I had a similar problem with the starter on my '82 300D. It seems that these Bosch starters, after quite a bit of wear, will not want to start when hot. A rebuilt starter solved my problem.

One of the things that confused my troubleshooting was that, as I checked various potential sources of trouble in the starting system, the starter was slowly cooling down. At some point it would start and would lead me to believe that I had found a bad component. I asked my indy about it and he explained the worn Bosch starter problem to me. Problem resolved....

Wes