Are you sure you weren't actually going between 70 and 80?
Assuming your cable is nicely lubricated and not winding up and releasing, causing the jumping you're seeing, let's think about the 'insides'.
These speedometers are the typical "eddy current" type, where the cable spins a magnet inside a "speed cup" which is connected to the gauge needle.
Here's an explanation of how eddy current speedometers work. In a nutshell, the faster the spinning magnet spins, the stronger the eddy currents are, which further turns the speed cup against its weak spring.
I know when I opened my CA95 speedometer, it was a mess. Lots of grit everywhere, and the original grease on the gears was missing or hard. With an earlier restoration, I encountered issues similar to yours , in addition to repeatedly snapped speedometer cables. For the past 3 projects, I've opened, cleaned and re-lubed the speedo, and each one has worked perfectly, although I completely understand how undesirable that task can be. Here's a picture of my CA95 speedo:
If the needle is jumping, that means the speed cup is jumping, too. I can see these possible causes:
1) Old grease not allowing the magnetic cup to spin freely, causing it to wind the cable until the tension is great enough for it to break free, causing the jump in the reading.
2) The cable is not lubricated sufficiently, causing it to wind up until it breaks free, causing the jump in the reading.
These speedos are no fun to open up, a process that consists of slowly bending up the crimped metal bezel and repeatedly stabbing yourself in the hand with whatever device was improvised for the purpose. If you decide to open it up, there are some good tutorials on-line, with tips on how to un-bend the crimp without creasing or damaging it. Otherwise, NoS speedos are available, too, but a 50 year old speedo could have hardened grease, too.