Under normal acceleration the trans should shift from 1st to 2nd at about 20mph to 3rd at 28-30 and overdrive at about 42mph, if it has lockup this should come in at 52-55mph. If at heavy acceleration the speeds will be higher, and if you're passing it will down shift as far as it needs to under heavy throttle to pick up the speed to pass. But if you're just driving down the road maintaining your speed and the vehicle bounces from 4th to 3rd or OD to 4th it could be a throttle position sensor, or a speed sensor.
