an 84 500sec and i have had the same problem for all the three years that i have had the car. i have had all the things done that you have and i still have the problem. for me, it has become a rather minor annoyance that i have adjusted my driving to fit since we havent been able to cure it. btw i think it is a problem that is characteristic of many cis cars from when they were relativeley new.
1. always depress the accelerator partway when starting. this was suggested by a former longtime mb mechanic.
2. leave it run if you are going to be only 10 or 15 minutes stopped. this works in lafayette but might not work in a big city. theft.
3. a recent poster replaced the check valve at the fuel pump and solved what sounded like the same problem on a 300e on this website recently so i am going totry that on mine one of these days.

good luck.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.