Alan
Sorry to say but this is one of the most prevalent problems with our SL models! Search this forum first, there are many, many posts regarding ASR problems - and solutions. Search under my name for some interesting reading about simply adjusting the cable - it's not always the throttle unit!
http://snipurl.com/atmn
The ASR system is designed to trigger if you rev the engine by the cable as the car doesn't detect the accelerator pedal being pushed - this is perfectly normal. This will set a code but depending on your production date may or may not remember it.
Have a read through Jim's site as it has lots of good techie info on the ASR system...
http://pages.prodigy.net/jforgione/MB_asr.html