1991 420SEL with 195k miles
This car has never had a "silky" smooth idle and recently the miss had become quite noticeable. About 10 days ago I substituted med grade fuel for the premium I have always used and, I'm sure by coincidence, the idle smoothed out like never before. The hicup/stutter step when taking off from a stop even dissapeared Suddenly today it stalled at a traffic light. The car restarted but would not idle, if I kept my foot on the peddle it would stay steady at 600 RPM and when I shifted into drive, I could accelerate but the second I took my foot off the peddle it would stall. This problem went away as soon as it came and now everything "seems" OK.

What's going on??!?

Could be varnish and other crud in your fuel distribution system. For the cost of it - go to Pep Boys and get a bottle of fuel system cleaner and conditioner and put it in the gas tank. As you drive - at first the problems may appear more prevalent as the crud is desolved and improve as the tank is emptied. I find that highway runs helps while the cleaner is in the tank. Just a thought and a lot cheaper than other things to try. I have a 560sel that had idle problems for 2 years. If the above doesn't work - email me and I will list you the things I did to cure my problem. After much hair pulling and thinking that my V8 would never idle silky smooth - I now have success. Don't get me wrong - the idle was fine - just a slight hic every couple of seconds or so - just enough to aggrevate.