1994 MUSTANG GT AODE TRANSMISSION PROBLEM - EXPENSIVE OR CHEAP FIX??? - 306 Foxbody

1994 Mustang GT AODE car has the transmission downshift on its own into 3rd or even 2nd while cruising in 4th (Overdrive). This problem can occur often, or sometimes not for a while. It is totally random. We test drive the car and determine what it takes to fix the problem. Does it need a new transmission? Or is it a simple $150 fix?