i have a 1967 Barracuda that has the same jumping sped indicator that jumps +-10mph. How did you lube the inner strand speedo cable when it is incases in a plastic shield?
You have to pull the cable off the back of the speedometer and the inner wire comes out.
Lube it with a light grease (like lithium or white grease) as it goes back in, and twist it just a little during the last inch or so to seat it back into the plastic gear in the trans speedometer gear housing.
Clip it back on the speedometer and there ya go.
Doesn't mean that'll fix it, but it's the first and easiest thing to try.
Sometimes the cable has a kink in it and it'll bounce till you get that out of it or replace the center drive cable part with a new one.
Then there's always the option of a whole new cable.
I had to get a longer one for the OD trans and I was really surprised at how inexpensive they are.
Of course the new cable isn't as hefty as the OEM but it still works just fine.