I believe I have the same problem with my Mercury Mystique as stated by owner with 169K. My 1999 Mercury Mystique has 105K. Recently it started to want to shift as I was driving on the highway or a kick back movement.
No one seems to know what the problem is--one business tried to sell me motor mounts and another without an inspection said it might be a transmission converter. I plan on taking to to a reliable auto shop for a diagnosis. I thought driving up an incline triggered the movement but it started to act up driving on straight level roads around town. The action first occurred on the highway. I do not want to send money on a part that is not needed.