The last two summers I've experienced this problem with my '91 300E. I tried something which seems to work. I start the car with the A/C compressor off. It works fine hot or cold.

I've been attributing the problem to the temperature but it appears related to the A/C compressor.

BTW, the car has always shifted hard with the A/C compressor running but just fine with it off. This problem has occurred since the car was brand new.

I can't explain any of this.