Check to see if water has gotten into the cylinder that is mis-firing. I had that problem with a 97 C280 after someone had "washed" the engine. Also check for bad plugs/wires/etc with the ignition system. If its a bad spark plug, that could indicate a bad cylinder, i.e. not combusting properly, fouling out the plug.

Good luck..

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K