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??!?