Speedometer not working properly

Welcome to 2CarPros.

Interesting technique, but it works. LOL Either you have a loose connection or there is a problem with the speedometer itself. Also, since the speed sensor is on the transmission and hitting the dash makes it work again, the problem has to be under the dash.

I looked through a few schematics and found the ground for several things, including the speedometer is located behind the dash (center). Take a look at the picture attached. Start there and see if anything is loose, corroded, damaged, or partially broken. If that is roughly where you hit it, that may be the issue.

Here are a few links that may be helpful:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

Let me know if this helps or if you have other questions.

Take care,
Joe