I have a 1991 tbird sc, it has a 3.8 supercharged v6 with an automatic/overdrive. When i put it in overdrive and go down the road it will either not go into fourth or it will go into fourth and stay in fourth as long as u keep on the gas. If u let out of the gas to coast or slow down it will drop to third immediatly even at higher speeds. The car has had a new tranny put in. It worked fine when i first put it in. Another more weird thing is when it is cooler outside it seams to shift a little more corectly but not completly. Has anyone else experienced anything like this, or has any knowledge of why this happens? Thanks, Cam

Have an experienced transmission mechanic make sure that your transmissions detent cable is properly adjusted. The detent cable is supposed to make the transmission down shift when you speed up while in overdrive. its possible that its not properly adjusted causing your shifting problem

Does a 91 have that cable?
I thought (and am probable wrong) that it was purely hyrdraulic auto trans?.

Check for leaks.
Check the trany oil.
Flush the Trany oil and replace it.