Coil(s) seems to be the common cause of this problem for M104 engines. Had the same problem in my '96 E320 a month ago and it turned out to be a bad coil. Ran perfect one day then started intermittantly missing on the way to work and progressively got worse.

I wonder if it makes sense to just replace the coils once in a while and what that schedule would be. As far as I know, the one that failed on me was original, so that's 245,000+ miles. Surely if they were replaced every 100,000 miles you could avoid having them fail.

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Brian Rogers
'96 E320 traded in at 292,000 miles
'09 C350