1984 Riviera Shifting Problem

My grandfather has an 84 Riviera he bought it new, it is in excellent shape has 70k miles on it runs ok. When the car is cold it shifts fine but after it is hot it shifts at odd times. For example when we bring the car back to his house it has to go up a grade before getting to the garage while going up the hill the car will upshift and then subsequently have to downshift because it is not going fast enough for that gear. Is there a sensor or some sort of adjustment that can remedy this shifting problem. I have checked the tranny fluid it is to the full line and looks very clean.

A loss of power can cause this otherwise you could check the tv cable adjustment .