Voltage rising then dropping? (67 Barracuda)

Good evening...

I've been having ongoing issues with my electrical system in my 1967 Plymouth Barracuda.

Recently there has been a ton of symptoms but I believe it has to do with the alternator.

If the headlights are on, the lights will start dim, getting brighter, brighter and brighter only for them to drop seconds later. The alternator is also making weird whirring noises and even the heater (if I turn it on) spins faster and then drops. It all coincides with the sounds coming from the alternator. At night, the dashboard lights also get brighter and then eventually drop again, too.

We changed the alternator already, along with the voltage regulator AND the battery. We scraped paint of the firewall to have a better ground, too and originally we got the car working again. But now it's acting up again.

The car does run... here's a video. Please check it out and listen closely. The first 4 seconds are a prime example.

If you have any insight, please share it with me.