The speedometer needle in my 1977 has recently started jumping up and down. The speedometer needle also makes a funny sound when it does. Then, randomly it will act normal again. It doesn't seem to matter how fast I am going or accelerating or not. Does anyone know what the issue could be?

 

it's binding up on something temporarily... couldnt hurt to pop it out of the tranny/tcase and spin it and see how it feels and check it for missing teeth

 

Update: I finally got around to checking out the speedometer cable. This was because the speedometer quit working altogether. The speedometer cable was burnt. It was resting against the exhaust manifold. So I ordered a new one. I replaced it. Then it worked great.