PO500. This code popped up on the wife's car recently. The speedometer never stopped working but occationally the trans would slip, and I found out why. The wire that runs to the sensor was fraid at the spot that touches the rear motor mount. Not only was one of the two wires exposed, the thermal heat sleeve the was around the wire was cut in two.
As you can see.. I took a short peice of speaker wire protector and stuck it on the motor mount after I fixed the wire and so far so good. Just a heads up to check your sensor wire. For once a Taurus trans is slipping and it's not actually the tranny.