Check the dwell.
If the dwell is erratic the tach will jump all over the place. I'm not driving anything with points at the moment. I've had cars that the tach would tell me when the points needed adjusted or replaced, just by the way it acted.
When you rev the engine up, the dwell should not change more than two degrees. Check it with and without the vacuum advance. You could even put the dwell meter next to the tach while you drive it.
