Seems like it happens when the car has sat for a while. When the car is cold, and you just get going from a stop, the speedo jumps from anywhere between about 5-15. Then, once you get above 20 mph, it stays that way for a while. Once it has been driving for a while, it works again. Any ideas?

 

If it's a cable driven speedometer, i suspect that the cable needs to be inspected and lubed.

fs