Has this problem just started? Have you done anything under the hood that would coincide with this? My first inclination is to check all vacuum lines for splits or cracks, or if one or more have fallen off their fittings. This could easily be a vacuum problem. On my S600, there was one vacuum line in the rear of the engine compartment that slipped off by itself a couple times. RPM would drop at idle, and the engine would hunt and run rough. If not, then see if there are any stored codes.
http://v12uberalles.com/pulling_codes.htm