Is your ABS warning light coming on? The speedometer sensor also supplies a signal to the ABS control unit. If the ABS is working normally, the sensor isn't your problem, nor is the wiring---the speedometer itself has failed.

The wiring connection to the sensor is prone to problems. Due to its location it can become corroded, and as it moves up and down with the suspension the wire itself can fray if it is rubbing on anything. The wire can also go bad internally. I had this happen once---the ABS light would come on occasionally, usually after going around a sharp turn, accelerating hard or hitting a bump. The speedometer would hiccup once in a while too. I replaced the harness extension from inside the trunk to the sensor and haven't had a problem since.