I had the same thing happen to over 2-3 months. I ended with a dead battery. I replaced the battery and the alternator regulator and brush assembly. Now all is fine. When the engine was cold, compression was low enough and the oil wasn't up to temp to help compression so starting was fine. When the engine was warm it took prolonged cranking to start. After replacing the bettery and alternator bits the starter turns faster and the car start right up when warm or cold.

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Fred Johnson
1977 240D
1983 300SD