It's definitely controlled by the ECU; even W124 OM606s idle speed is closed loop controlled, never mind the fully electronic pump on a W210.

I would begin by checking the operation of the throttle pedal position sensor - you'll probably need to plug it into a star tester, and read the live data. Before spending money, though, make sure that the throttle cable hasn't been over adjusted - i.e., make sure the TPS is going right back to idle position with a little slack in the cable when the pedal is at idle position.