The shifting of the transmission is controlled by the valve body in the transmission and the vaccum modulator, sounds more to me like a valve is sticking in the valve body if you have service the transmission on a regular basis with a fluid and filter change that probably isnt the problem, the vacuum modulator is controlled by engine vacuum and could be bad, they usually run about 7 to 20 dollars. I would recommend checking vacumm to the modulator and replacing the modulator before having the transmission rebuilt.
