RPM gauge bouncing and stalling randomly in gear and in park.

Thanks for the video make my job much easier. I would start by changing out the ignition coil w coil wire, it looks like the ignition system is back feeding into the tachometer circuit. Here is a guide to help you change the coil out:

Ignition Coil

Remove or Disconnect

- Make sure the ignition switch is "OFF."

1. Electrical connectors at the side of the ignition coil.
2. Ignition coil wire.
3. Nuts holding the ignition coil bracket and ignition coil to the engine.
4. Ignition coil bracket and ignition coil.
- Drill and punch out the two rivets holding the ignition coil to the bracket.
5. Ignition coil from the bracket.

Install or Connect

A replacement ignition coil kit comes with two screws to attach the ignition coil to the bracket.

1. Ignition coil to the bracket with two screws.
2. Ignition coil bracket to the engine bracket with studs and nuts.
3. Ignition coil wire.
4. Electrical connectors.

Make sure the coil is tight to the intake manifold and make sure you use AC Delco ignition products. Check out the diagrams (below). Please let us know what you find. We are interested to see what it is.

Please let me know.

Is the coil tight to the intake? Also did you replace the coil wire, distributor cap, rotor and plugs wires, spark plugs, If any these items fail the coil will bounce like that.

Here is a guide to help:

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

Please run down this guide and report back.

All AC Delco? I would double check everything becasue a cracked spark plug can cause the issue with the tach. I have never seen a tach do that on its own but there might be a first time. If everything is good you will need a rebuilt cluster here is a video to walk you through the repair

https://youtu.be/_HEC44xENxw

Please let us know what you find. We are interested to see what it is.

Sounds good let me know.