93 Taurus Transmission Trouble

ok my dad has this 93 Taurus. with the 3.0 V-6 and an automatic. i never really work on automatics so that's why i'm asking. when the transmission is below normal operating temp(like right after you start it) it's fine but when you get to normal operating temp after driving it awhile it'll slip out of gear then go back in and hop. and then coming to a stop it'll slip completely out of gear and you have to put it in reverse at a complete stop, wait for it to go into gear then put it in drive and go. this happens everytime you come to a full stop. i'm not sure what it could be. we ran a snap-on scanner on the car to try to get a diagnosis and it says the speed sensor but i'm wondering if it could be an internal problem? and my dad's flushed the trans and refilled it. and we've tried everything short of an overhaul. but there's supposed to be 2 speed sensors on the particular trans. and he has not changed them because the scan tool does not say which one is bad. just that there's an error. anyways i'd really appreciate any tips or advice you may have.

thanks Ian