I would first replace the distributor cap and rotor unless they have been replaced in last 30k miles or so. It sounds like your mechanic may have already done the EHA adjustment, but if not I would try that (do a search for EHA and you'll see what I mean). I would also look at this link:

http://www.mbca.org/pages/tech/MBCA_Rough_starting.htm

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car