Tighten alternator belt. Clean batttery posts and cable clamps along with the ground conn on engine. Make sure ground straps are from eng block to firewall, i.e. valve cover bolts to bolts on firewall.
Take a voltmeter and measure charging voltage at battery. Should be 13.8-14.8 volts. Turn-on headlights, should drop to 13(?) volts. Rev up eng to 1200 RPM, should come back up to 14 volts. Nothing wrong, just the ext regulator is not doing its job 100%.
You can choose from two options:
1. If you like to keep the stock & original look, then buy an aftermarket "electronic" regulator and replace the regulator.
2. Or more costly, replace the regulator and alternator with a one-wire alternator.