Help Please

Hi, I have a 1994 Ford Thunderbird 3.8 V6, Auto Trans. While driving at any speed the car keeps changing gears, even if your going 60 MPH in overdrive, it will still go in and out of overdrive. If your going 35 MPH it will go in and out of gears. This occurs on hills on flats and even with the cruise control on. ANY IDEA WHAT'S WRONG?
Thanks
Joe

it might be excessive drag somewhere causing the transmission to think that it has to shift down.

Try jacking all the wheels and see if they all spin properly.
Also check your tire presure