Your problem sounds just like the problem I had in my 56 210. The needle would start bouncing around and making an awful screeching noise. Here is how I lubed the speedo in my car: (your truck is probably similar)
Remove the speedo assembly from the truck.
Where the cable screws onto the speedo, look for a little brass cup in a hole. Dig out the cup with a pick or small screwdriver.
Inside the hole will be a little piece of felt. Take it out and clean it with a solvent and dry it completely.
Put a few drops of machine oil on the felt piece, and a couple of drops directly into the hole in the speedo. (most folks recommend sewing machine oil for this, I used Hoppe's gun oil. The colorless oil. Not the Number 9 solvent.)
Replace the felt piece in the hole and the little brass cup.
Use a small drop of oil to lube any other moving parts. Wipe off any excess.
Then you can put it all back together and the noise will be gone.
56chev
