Date registered: Feb 2008
Vehicle: 2004 W220 (S430) ; 2001 R129 (SL500) Sport
Location: Alabama Gulf Coast--formerly Charlottesville, VA
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If it were a V-8 version, I'd say it was a crank position sensor. I don't know the 6 cylinder versions well enough to say that with confidence. The rough idle may indicate MAF troubles either as well or as the sole cause of the problems. I'd suggest you take it to a place that can read the codes from the OBD II port. Perhaps that will tell you exactly what the problem is.
Sorry I can't be more confident or specific.
"Don't pick a fight with an old man. If he is too old to fight, he'll just kill you."--John Steinbeck