Idle speed
 

My s280 w140 idles at 750 rpm when in park or neutral. This is fine, but when i engage drive the revs drop to 500 which causes problems. When i stop on a hill the car does not stay still but starts to roll backwards and i have to feather the throttle. I have been told that it is the vacum modulator on the gearbox, can anyone give me some advice as to what might be causing the problem.
Many thanks
Lester.

What year is the car? The 722.6 transmission will allow the car to roll backwards down a hill in gear 1. Does it run smoothly when at idle in gear?

Idle speed
 

My car is a 1995 and the engine does run reasonably smooth although 500 rpm for most engines is asking a lot.
Many thanks,
Lester