If you are seeing arcing that was the time when it was Neccesary..since then you have been getting far from ideal combustion. With wires sparking I would bank on a full replacement. Wires (use Beru) cap, rotor and plugs. Do NOT use platinums.

Jonathan

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Blue Ridge Mercedes
Jonathan Hodgman
http://www.blueridgemb.com/
Enthusiast Service, Restoration & Tuning.
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Located in the Atlanta area
Specializing in all pre and post merger AMG's including Hammers and DOHC M117 engines.
Mercedes Repair Atlanta