macarthur

I have a '69 cutlass convertible with a 350 rocket and 350 trans. I am starting to have some issues with the transmission when the engine gets hot,, while accelerating it tends to want to stay in the lower gear and not shift when going around 50 mph or higher. Any suggestions as to what it may be ???? it shifts good when the car hasn't been run and is cold,,,,,,, any suggestions on what this could be or is a rebuild in order ??? thanks in advance !!