fluctuating ammeter

i have a 1970 340 dart that when im driving the ammeter bounces from high to low it seems to happen once i get revved over about 1500 . so far i tried a different alt,ignition switch ,reg,starter.battery. so now im pretty well out of ideas .im redoing a bunch of stuff this winter and hopefully some upgrades but i would like to fix this also it does run fine but it doesnt seem right .i was thinking of a painless wiring kit also dont know if it would help or be worth it. any info is appreciated thanks