Tach on a 72 VW

What would cause a tach to jump sporatically and redline too fast before the rpm’s are even high. Thanks

Does your tach need to be set for the correct number of cylinders?

I’m assuming it’s using points, still. Are the points in good working order, and properly gapped? Use a dwell meter and set it to spec- feeler gauge is remarkably imprecise in this application.