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Hi Eric, welcome to Just Answer. My name is***** sounds like a fault with the electronic throttle actuator. When the actuator malfunctions the idle can be erratic. When you accelerate hard the actuator can default to a limp home mode if a malfunction is detected. When you turn the key off and back on it will reset the actuator and everything will work fine until the fault is detected again. I suggest having the car connected to a Mercedes tester to check for fault codes stored. I would also suggest having the actuator inspected and cleaned. You may need to replace the throttle actuator to repair this problem.
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You need to get the fault codes to be any more specific but this sounds most like a bad throttle actuator. The pedal would not cause this issue.
Yes, that is the part I am referring to.
I am referring to part number 113 141 01 25. Click on the image below.