IS THE IDLE THAT BAD THAT YOU NEED TO SHOUT JUST TO BE HEARD? NO? WELL MAYBE YOUR CAPS LOCK IS STUCK ON!
Anyway... seriously.... my 300CE had the same problem, and I changed the motor mounts, and the cleaned the electrical contacts for every compter, relay, fuse, sesnsor, ground lead etc.
Totally cured the problem, and I got an increase of about 2-3mpg.