| Remove the pickup on the right side of the tranny at the top and clean it off. If it is covered with small particles it totally messes with the speedo. If that does not do it, check for loose connections under the dash on the harness etc.
Using your service manual (you do have this indispensable tool right?) there are several simple tests that can help you verify the operation of the speedometer. Start with the pickup as it is the most common cause of problems. Also, if it does help change your tranny fluid out to help reduce the particles in suspension. | 
