I'm no expert in MB, nor do I know your car very well, but I would check the fuel delivery and return, perhaps something is getting stuck closed and results in all the fuel which should be returned getting dumped into the cylinders, this popped into my head after you mentined that the car works fine on the highway and starts acting like this when you get off the highway.

xp

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1985 190E 2.3L - a constant project.