You need to check the fluid level.
Could be torque converter going bad or cylinoid in a certain gear is burning up and going bad.
the transmission could be going out or slipping or your transmission fluid could be low
sounds like the transmission is slipping
If the whole car feels like its bucking it could be the transmission is slipping. Either its going bad or you need to get new transmission fluid. Sometimes older cars, especially automatics, do that if you put in a lower gear other than drive.
we have a 2000 astro van that has been slipping, wants to die, and the repair for it is a new torq converter on the transmission, actually a rebuild on the transmission because of the torq converter. rather expensive repair.
most likely your transmission is slipping and usually is the onset of your transmission going
A slipping transmission is like driving on ice or trying to get away from the monster in your dreams. You are running like mad but not going anywhere and when you wake up you are still in the same place but are overheated. Change your fluid and transmission filter.
Your clutch might be going out if the transmission seems to be slipping. I would advise bringing your car to a garage before further damage or even an accident results.
In my experience as a non religious person when mine started slipping I resorted to prayer! After reading countless reports on how horrible this transmission is and it had been recalled at 30k miles. It is most likely going to be very expensive and may need to be replaced. Sorry to deliver the bad news. If you have different experience please post it.
it starts slipping, highly recommend taking it to Ford service the sooner the better, less damage to the internal parts. Easy transmission for 2nd rate shops to mess up on.
if their is no slipping or any other transmission issues and it is just only the loss of overdrive try replacing the brake light switch
sometimes it can come from your transmission going out or from some kind of gas censors.