Try fuel injection cleaner, Chevron WITH TECHRON. Put it in with about half a tank, and don't give up if it doesn't work right away. It may take 4 or 5 containers over several fill ups before it works itself out. I'd guess you have a dirty injector or fuel distributor. It's relatively cheap if it works (let me know if it does - I have the same problem and was recommended this remedy)
Fuel air mixture is adjustable, but I believe it's computer controlled also. The idle control relay is more expensive to change so I'd leave that for a last resort, I'm not sure that it would solve the problem. If the techron doesn't work.
2003 S500 Black/Charcoal
1990 560SEL 61k Arctic White/Grey
1988 420SEL Black/Palomino Sold @ 85k
1987 420SEL Midnight Blue/Grey
1986 420SEL Diamond Blue/Grey
1983 380SEL Champagne/Palomino