Are you still using points or an aftermarket electronic ignition module?
My 67 tach displays approximately 150 RPM's higher than what the engine is running at but its movement is smooth at all RPM's and using points.
Recently I had intermittent stalling and when it was running between stalls the tach would be bouncing and way off in RPM's vs. motor speed. I eventually found the cause to be a loose post nut on the coil that went to the points.
Mike