1991 B350 Speedometer problem HELP

From : z1kid

Q: my o/d lamp lights and the transmission comes out of overdrive while driving at highway speeds typically 60-70 mph. this behavior just began today i am not towing and the ambient temperature is around 50 degrees. one time the transmission hot indicator came on but went out after i stopped turned off the engine and restarted. if i stop and turn off the engine then restart the o/d indicator goes out and the transmission works fine for probably 15-20 miles then the routine starts over. the transmission has plenty of fluid still red but i do smell that telltale hot something smell when i walk past the radiator. i have never changed the transmission fluid/filter. should i have the dealer change it and adjust the bands as the owner manual says any other ideas .