You need to have a good mechanic pull any stored fault codes from the car. It is very difficult to diagnose these cars without knowing what fault codes might be present. Keep in mind though that many items in this car are tied into each other. The failure of one part can create false fault codes. The ETA (Electronic Throttle Actuator) could be bad. Wiring insulation failure is a wide-spread problem. In addition, there are potentiometers inside the ETA's that wear out with time. That will also cause all sorts of problems. These links may help...
http://v12uberalles.com/throttle_actuator_rewire.htm
http://pages.prodigy.net/jforgione/MB_asr.html