For what it's worth... I had a very similar problem with my 1994 SL500. It turned out to be a bad "EA/ISC Actuator". It's essentially the throttle body assembly. There is a switch in the assembly that tells the computer when it's supposed to be at idle. The technician was able to read some fault codes that indicated that it was a problem. (Something like "lost communication with..." Not sure it's the same thing... but the symptoms are identical. After replacing it.... I still have a somewhat rough idle that seems almost like something is out of balance. But that's a different discussion.