Tranny is slipping or your detent is hanging up. Detent keeps the tranny in a lower gear longer or is what causes your car to downshift for passing. I'm not up on these newer trannies though as to whether they use a cable, solenoid or electronics to downshift. Just something else to look at before going through the tranny. Get you a book on the car or go to the library to see if you can find a trouble shooting guide on the tranny.