[scottbooth]

It could be the sensor on the transmission, though mine failed all at once (not intermittent/heat related).

Does the odometer stop counting when the speedometer reading drops to zero? If so, it's probably the sensor or the wiring. If not, the problem is likely inside the speedometer itself.

On my 78, the sender connects to the wiring harness inside the tunnel, under the access panel between the rear seats. The sender has a thin coax-type cable. Unfortunately I can't tell you about the connector because the PO of my car had removed mine and spliced the sender directly in. On my car, it was the only electrical connector in that area.

To test the wiring harness and speedometer/odometer, you could unplug this connector, switch on the ignition, and then use a wire jumper to repeatedly short the connector pins on the wiring harness end. The "sender" is just a magnetic switch that closes several times for each wheel revolution, so if you can mimic this pulse pattern by shorting the pins one or two times a second you should see the speedometer needle move. I did this when my sender died years ago, just to make sure before I ordered a new one.

One way to troubleshoot this would be:
1. Confirm that you can get the speedo needle to move using the above method (ignition switch on, engine doesn't need to be running).
2. Reconnect sender, then go drive the car until the speedo stops working.
3. Park the car, leave the ignition switch on, and repeat the above test. If the speedometer "passes" the test, order a new sender.

Good luck
Scott