1999 Taurus intermittant speedometer and shifting problem

I have a 1999 Taurus SE with a 3.0L engine. My problem is at times my speedometer will stay at 0 and the tranny will not shift out of low gear when that happens. Then in a little while the speedometer will jump to normal and the tranny will shift fine. One time the code P0500 showed up so I replaced the speed sensor that is located on top of the right side of the tranny but it didn't fix the problem also wiring to the speed sensor is clean. The same problem still happens. Does anyone know what might be the problem? Or is there another sensor that might be causing this? Thanks