MB Jerks at 20 MPH to 40 MPH when the RPM is Less than or at 2,000

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My car jerks when I drive it on automatic between 20 MPH to 40 MPH if the RPM is at or around 2,000. If I hit the gas and the RPM goes at or above 2,500, the car DOES NOT jerk. Sometimes, when I am at 40 MPH and I slow down to about 30 MPH, as soon as I step on the gas it feels like the engine thinks I am too slow for the 4th gear so I shift down to 3rd using Steptronic and all is well. Can someone please tell me if it a computer on my MB telling my transmission the wrong thing? Thank you!