It doesn't feel like a misfire, but it just kinda revs up then down at idle when it is warmed up. still runs smooth, but just "hunt" or "surges" instead of staying at a constant RPM.If anyone has any ideas or experienced this with theirSL600 let me know what it was in your case. Thanks.

 

Had a very similar issue with my 500SL. Turned out one of the ignition coils was faulty. Could be the same problem for you?

 

Vacuum leak,check the rubber between the airflow and ETA's.
Crack wo'n be seen unles removed and inspected.
Good luck.

 

Put a scanner on it, you may get a P0507 code.