I don't know if you already scanned, because if you don't have fault codes, it's a bit more difficult to give a diagnosis without trying the vehicle. Sudden changes occur for many reasons. High line pressure, bad speed sensor, absence of signal signal or speed speed output signal. If the changes make them at the right speed and hit it could be line pressure, your first one you have to test the transmission and have the data of the manual in addition to being sure that not having much game when assembling the disc packets.