My very first post involved trying to solve this problem. Trouble was, it was so intermittent that I couldn't define when it occurred.

Other more serious problems cropped up since, so I had to deal with those first. Problem went away briefly, so I thought it was just a fluke, but it's back again with a vengeance!

Since I have taken my W124 offline to "exorcise" it of its little demons, I though I would try to attack this problem once more.

Idle is fine when transmission is in neutral. When I shift in "reverse", the idle cycles forcefully between 500 and 1000 rpms...if you look at the car from the outside, you would think I was having trouble engaging some "low-rider" hydraulics...

Click it back to "neutral", all is well.

This sometimes occurs when in "drive" but not very often.

If I need to look at vacuum hoses, which ones should I be checking? Some of them I've seen described in other posts don't seem obvious...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car