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Does the engine EVER run smooth at idle, like cold or hot? Or damp days or dry days? Have you tried driving it without the fuel tank cap on? What about misting the plug wires with water in the dark, any external sparks you can cordinate with the misfire? Is the spark plug you mentioned the correct plug for that engine? I know that trying to upgrade plugs, especially Bosch, is NOT a good idea in a Mercedes.

I am betting it is the ignitor, which I believe has a tired capacitor in it.

Good luck, and be sure and let us know what fixes it!

Jim in Phoenix