I am sorry that you have not been helped yet. I just came online and saw your question
Ok, this may be the electronic throttle body, the APP(accelerator pedal position) sensor, computer, or short in the wiring harness between the 3. If you have a MB STAR scan tool, I can walk you thru the testing, otherwise, unless you want to spend alot of money throwing parts at it, you need to have a Mercedes shop diagnose this.
I understand that you likely were hoping to hear of a simple and easy solution not requiring complex testing equipment, but unfortunately that is not the case here.
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