verify your spark plugs are the original model as ORIGINALLY specified for your car and as specified in your owners manual. Mercedes dealerships or the local parts store often have substitutions for part numbers for the plugs. They get the model or the resistance wrong and MB will substitute an inferior and incorrect plug to simplify dealership inventory while wreaking havoc on these older cars. This was the case for my 1984 380sl by substituting the w9dc0 for the correct w9dc or the local parts store incorrectly specifying, in their parts feeds, plug model numbers.