1965 Chevy C10 - instrument cluster blues

I was riding down the highway and looked down at my dash. The speedometer needle is just floating back and forth, back and forth... and the odometer went to roll to 80000|9 and stopped.

Hmnn.

I haven't checked the cable or anything yet, as it was dark when I got home.

Any ideas on things to check folks?

Thanks.

-W