Speedometer is acting funny on my 1998 Grand Prix. Why?

Question

When driving, my speedometer bounces between 0 and 70 when I drive on the highway. Any ideas of what is causing this?

Answer 1

Bad cable/no damping in speedo, bad connection if electric/faulty speedo

Answer 2

I have an 89 Sunbird that does this. I assume there is a bad ground somewhere because if I push the entire cluster inward it goes back to normal. I think this has haunted Pontiac for decades.

Answer 3

Most likely a bad speed sensor or speed sensor drive gear in transmission. Gm also had problems with there instrument clusters s******g up and you have to take it out and send it off to have it repaired. I would think speed sensor first and check the wires at the sensor too.