JD-
Randy had the right suggestion. The idle control unit is likely the next thing to replace. I've got a few from the local yards-- they run a couple hundred new, but yards have them for generally $20 or $30. If you have a friend with a w126, you can swap out to check, but that's the only way to "test" these things that I know of. I can send you a known good one.

Beyond that, it's a bit of a pain to track down a high idle, but vacuum leaks are the most common culprit. Replacing the air distribution lines is what finally fixed my problem. Around $100 for those from ********AZ.

Here's a link to look through:
http://www.slack.net/~thundt/mercedes/high_idle_hell.htm

And this-- look at posts #63 and #64
http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1340751-do-yourself-diy-repair-thread-wheel-2.html

Good Luck!
Dave