T-bird Tranny

I Have a 1990 Model Thunderbird. The tranny is acting funny. I have just replaced the engine in it and now fear that I will have to do the same with the transmission. It drives fine until you reach speeds of fourty-five to fifty mph. then the car seem almost as though it is slipping into nuetral and begins to decelerate. upon reaching fourty-five mph it kicks back in. Some say that is the overdrive and others that the bands need adjusting is there a quick fix for the car or do I have to replace the tranny?:confused: