replace your plugs with the correct bosch plugs (H9DC or H 9 DCO depending on manufacture date) for $1.75 each and your problem will go away. Do a search on "spark plug and resistor" and you'll find information as to why the platinum plugs do not work well with Mercedes ignition systems. Other techs can comment about this as well..

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Daily Driver: 02 E430 4MATIC
In the family: '03 E500 // '04 ML500 // 64 220SE