I have the exact (not similar - exact according to your description) same problem and no one has fixed it for me. I have been to four different Mercedes Dealers, one of which told me to junk the car (they're nuts). All the ignition checked out fine, including cap and rotor, coil and wires, etc.) Cold start system and O2 sensor are fine. I am taking it to my independent mechanic tomorrow night. He is going to specifically check for vacuum leaks and fuel delivery problems. He is also going to check out the valves and stuff. My plugs in #4,6 and 8 are fouling so I suspect too much gas in each of those cylinders. They will keep the car overnight so they have a cold car to start in the morning and I expect to have an answer by Friday at the latest. I will let you know what they found.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired