2000 Hyundai Tiburon speedometer stopped working

my speedometer stopped working and then my transmission started shifting weird...... it shifts fine when you first start driving but when it warms up it downshifts and then wont shift back into overdrive... but if you pull over and shut it off and restart the car it works fine again for a minute then messes up again
.... could that be because of the speedometer not working and it sending signals to the cars comp and make it downshift because it is readint no speed.... how to i fix this?