Having had the same problem (idle fluctuating between 500 and 1500) on my 1986 300E, having read all the same threads you're reading, and then having fixed the problem by reconnecting a vacuum line that came loose from the intake manifold, I reached the following general conclusion. It appears to me from the many different threads with the same initial symptom but different resolutions, that the fluctuation is a very general, catch-all symptom. It seems like the fluctuation is just the natural behavior of the fuel injection system hunting for the right mix in a system that has some flaw that keeps the mix from ever being right. Having noted that there were many *possible* things that could foul up the mix, I looked for the easiest, cheapest one, ergo, my vacuum leak. I hope your luck is similar. Mine was aided a bit by the recollection that the problem started after I'd done some other work, so I started looking for loose hoses in that vicinity.