1993 Chrysler New Yorker Transmission Dilemma

The problem I'm having is that when the car (transmission) warms up good, the transmission begins to go haywire. Coming to a stop it will downshift really hard, and when resuming normal driving it will not shift into any higher gears at all no matter your speed. Now if I pull over and put the car in park and turn off the ignition for a minute then restart the car and shift into drive it will drive normally until I stop again. Then the hard downshift, and not changing gears etc. Here's the wierd part, occasionally the speedometer will start jumping erratically and eventually sit at 0, no matter how fast you're going.
My son has a 2001 Dodge Intrepid and is having the exact same problem, and has replaced the following parts, Input speed sensor on the trans, PCM, generator, and had the transmission fluid and filter changed. None of these have fixed the problems. Hope you can help.