65 Ammeter now shows overcharge when accelerated

So my mechanic told me my alternator was charging but not engugh at idle. It was 11.70 volts. He went through and checked all the wires, I swapped out the alternator and it still was at 11.70. He told me the regulator could be bad. I bought a new one from Vans Auto and installed it today. Now it's charging 12.50-12.80 at idle 600-650rpm. When you rev the engine up or drive it the Ammeter gauge goes from the middle to almost pegged to C like it's over charging. I tried it with the volt meter and it was at 13.40-13.88 with it revved up to about 1500-2000 rpm. So that seems right? But why is there so much movement of the ammeter gauge? It's an NOS ammeter Instruments Inc replaced my burned out one when they redid my gauges.