I know it's a totally different animal, but on my E420 (M119 engine), there are two vacuum lines that run across the front of the engine, right behind the air pump, over to the EGR circuitry behind the right front headlight. I was messing with my air pump a while back and noticed these lines were old and brittle. I know you checked for leaks by spraying, but these lines are far enough away from the engine that you might not notice.

On my car, at idle I see about 18" of vacuum. Are you getting any codes? Could be a bad MAF sensor too.

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