Hi All,
Thanks for the posts about high idle. I had same problem with a 1990 260e w124.
It was idling at 1500 RPM on start-up and sometimes dropping back to the 700-900 after driving for a while.
I removed the air cleaner to look at the linkage and the micro-switch. The micro-switch roller was partially damaged and riding up over the linkage part that contacts it. I was able to bend the linkage slightly to make it push properly on the micro-switch roller. Problem solved. Will replace micro-switch because there is worn area on the roller.
Because of the help from this thread I was able to go there straight away!!.

Thanks