Irritating mystery!

Have a 1991 300SL (54K). When starting out for the first drive of the day (but not always or predicably), the RPM's will drop to zero when I remove my foot from the gas. I can be doing 60 MPH! This only happens after the automatic choke quiets down. While still moving, a slight press on the gas will make the RPM's pop back up to normal and all is well. If I come to a stop without applying gas the car will stall. It will start if the gas is pressed to the floor. If I try to start it without pressing the gas it will not start until I floor it.

After about 10 minutes of this foolishness the car settles down and it doesn't happen at all the rest of the day! I can drive it during this 10 minute period by braking with my left foot and keeping slight pressure on the gas. Sort of looks like the problem kicks in about the time the car reaches normal running temperature and goes away after about 10 minutes.

This only happens periodically. The overload protection relay and oxygen sensor have been replaced. My independent mechanic can only think of throwing more parts at the problem. The dealer looked at me blankly and said I'd need to leave the car with them for a few days so they could see if they could reproduce the problem.

I can't imagine this is the only 300 that has experienced this problem! Any experience or ideas I can build on?

Thanks!!

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Jim