I had the exact same problem on my 94 E420. I thought it was the ETA, so I replaced it and still got the problem, intermittently. I lived with it by shifting to neutral at stops so the engine would settle down.

A few days after replacing the ETA, I replaced the Neutral Safety Switch, but since the problem was intermittent, I honestly can't remember if the problem eventually went away by itself before replacing the NSS or if the NSS actually solved the problem. In any case, I know for sure that I haven't had the issue since replacing the NSS.

If you are thinking about throwing parts at the problem, I would consider replacing the NSS first. I did have a similar problem with my 400E (surged in reverse only) and the NSS did solve that problem.

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles