Bouncy needle on speedometer

So just had my speedometer assembly rebuilt for $170. Installed and connected the cable. Needle shakes/bounces as it gets to about 35mph, then it smoothes out and is stable, then as I slow back down it starts shaking again? I called the speedo shop and he says the cable is faulty? Does that make any sense?
He also said not to buy a cheap cable, and to have someone make me a new one.
help?
66 dodge dart /6
auto 904

Out of curiosity what shop did the speedo for you?

Redline and North Hollywood seem to do a lot of Mopar stuff and people seem pretty happy with them.