I recently purchased a used 2014 MDX with 45k miles on it. If I am driving between 40-50mph the car jerks badly. I either have to take my foot off the gas or gun it to get to a higher speed. I brought it back to the dealership shortly after buying it and they said they couldn't find anything and everything looked and tested okay, but it is slowly getting worse and driving me nuts. Any idea what this could be? Hoping it's not an arm and a leg to fix...