Just had the same issue. Changed both speed sensors, no help. Eventually i got dead pedal, the whole cluster stopped working. Changed the batteries, cleaned all the connections to the pcm, and cleaned the grounds, and everythings fixxed.
Did you check the batteries? If one is sucking power the pcm won't get enough power to function properly so i'd start there. And there's another sensor on the side of the trans, but that really shouldn't affect the speedo.
