As a first step, lube the cable.
You'll have to get behind the instrument cluster and unscrew the upper end from the back of the tach. Once done, you should be able to dribble oil into the sheath. You may have to disconnect from the distributor and pull the cable through the firewall grommet to do this.
If that doesn't work, the driven gear in the distributor may have worn out. It might be plastic and I have no idea if they're still available.
There are several 510-specific web sites that can help with more details.