MAD ELECTRICAL?

My two cents worth: if wiring were causing your fluctuations, you would have a lot of other problems, like smoking connections or the car wouldn’t run right, or other electrical things also would malfunction.
If you can see it in the headlights, that means it is really happening and not a voltmeter issue. It is possible that you could have made it worse by improving your wiring. If you follow Mattax’s water analogy, if you install a bigger pipe you will move more water faster. A bigger wire from the alternator might make the fluctuations more visible.

I agree with mattax, I think your charging system is the issue. An alternator can have some burned out diodes and still operate but not correctly. A regulator, even a new one can be defective. One can even take out the other. Having the alternator tested would be a good idea. If you replace it, change the regulator too.

The wiring is different on the old mechanical regulator and the solid state one, unless it is designed to as a drop in to directly replace the old mechanical one.

The charging circuit does not go through the ignition switch, so I think that would be a long shot.

If it goes away when you rev it up, seems almost certain to be the alternator or regulator.