I've got a problem with the speedo on my '94z. Whenever the car has been sitting out in the sun the speedo wont work right, it will give random speeds. It doesn't stick, it just won't give the correct reading. Could it be a sensor? All the other guages work fine.

 

Some possibilities are the connector for the cluster is not making good contact or the speedo, itself, has problems. Also the wiring from the pcm to the cluster and the pcm, itself. You could possibly compare the cluster reading to a scanner to see if the pcm is sending a bad signal or if it is being picked up bad by the VSS on the tranny.