Amp gauge problem

The amp gauge in my 70 Dart is working like a tach.Going up and down instead of holding steady.After a while it will settle,but it still doesnt seem to be working right.This gauge shouldnt be jumping back and fourth like it is.

Things I have checked/changed:
New alternator
new Voltage reg
Checked grounds

Anybody else ever have this problem?
Is it just a bad gauge?
Anybody with a fix?

I know about going to a volt gauge and that is a possibility,but some things I like keeping stock.

Thanks