Nokan -

You may also wish to check your Idle Control Unit (found up behind the glove box on my 500SL - not positive about that for the SEL). I had the same symptoms and that was the culprit (lots of posts on this). You wouldn't think so if it is intermittent but it was.

They're several hundred $ - if you know someone with the 117 engine, bribe him/her with a couple of beers to swap/check. Also, get sme Doan's pills or hire a contortionist to get at it for you (LOL).

Good luck
Kevin