When waiting at a stop light, car idles very low and it almost shuts down - why? on 1999 Mercedes-Benz C280

I have to set the transmission to "Neutral" while waiting at a stop light, otherwise the car may shut itself off or it shakes violently threatening to shut down. What can I do to have the car idle when stopped, and on "drive," like it does when in "neutral?" My interim fix (setting the transmission to "neutral," sometimes leaves me unable to get the transmission to engage when, as the light turns green, I set it to "drive" again and wish to move.

Asked by for the 1999 Mercedes-Benz C280
It could be many things, unfortunately and without experiencing the problem first hand or being able to do some basic diagnostic tests its hard to say. It could be a vacuum leak, and engine that requires cleaning the throttle body, spark plugs and a "tune up" or a problem in the electronic idle control system.
Thanks for the Greg. I agree, I suspected it could be a variety of things. For some reason, becuase it is a Mercedes, I suddently feel clueless and aprehensive to dive into it as I've done with all of my previous automobiles. BTW, I checked your website ( - I liked the original spelling of the word "Excellence" by your picture. Thanks again Greg. Merry Christmas!