That was the exact same problems I was having at one point. Car would seem to jerk forward and backward a bit between 35-45 mph. I took it to the dealership and they put in a new trasaxle under Premium Care ESP.
I am not saying this is your problem though. I don't really think they know what the hell they were doing. They said the torque converter was failing.
What kind of car do you have? ATX or MTX?