My oscillating (500-1200 RPM) and sometimes steady high idle (1200 RPM) where solved after I cleaned the idle control valve.
Mind you, I had to clean it 3 consecutive weekends before it was solved (one can of carburetor cleaner and one can of brake cleaner). Problem has not happen in over a month since cleaning it the last time (hence my assumption that it was the cause of the problem). Also make sure you lubricate it with WD-40 when done cleaning.
To adjust the mixture you will need a multimeter with duty cycle capabilities (US 30.00 in SEARS). Place the positive pin #3 of the diagnostic socket, the other in ground (or pin#2). Properly tuned it should oscillate around 50% at both idle and 2500 RPM. A deviation of 10% from idle to 2500 RPM is acceptable.
Oscillates 40-50% at idle
Oscillates 50-60% at 2500 RPM
This I learned from SteveBFL, credit him for this info.
Without people like him, Benzmac, and every single contributor of this forum, I and many others could not be able to afford a Mercedes Benz (at least the repairs).
1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car