Dodge: 1996 dodge 1500 auto transmission, dodge 1500, foot on the gas

Question
While driving on the highway, it will shift from overdrive to 3rd or 2nd gear.  Sometimes it will do it while driving on side roads up to 50 MPH. When it does this it will not idle either.  This is an intermittent problem.  I notice this when it is hot or cold.  Transmission fluid level is correct.  Transmission shop stated it needed overhaul.  Only 2 years since the last one.

Answer
When this happens, is it quick between the shifts? Next time it is doing it, try holding your foot on the brake slightly, while keeping your foot on the gas and see if it levels out and quits.

What do you mean by it wont idle? At highway speeds it shouldn't idle.