Having digested 1000's of posts here over the years, I've learned the following:

MBs from your era are equipped with what's known as the M103 motor. Many had a small elect. resistor that provided an additional 50-75 rpm(approx) with a/c on. Also, there was a speed sensor on the back of the a/c compressors fitted to cars of this era that when faulty required the ignition cycling you're employing. Make sure your serpentine belt is snug as well - just a side item.

I do not recall the name of the idle boost resistor(my words) but perhaps with this you'll be better equipped to do more useful searching.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"