Question about a voltage regulator ...

[Jeepers Creepers]

Hey y'all,
Check this out. Update on the electrical issue and it's a real head-scratcher.

Here's the current sitchiation. If I turn my headlights on, they stay on for about three to five seconds, then they start flickering and the dash lights flicker too. The brake lights don't come on either, but that may be a second problem, and the taillights don't come on when you turn on the headlights. They did work before all this nonsense. The turn signals do work, but I did notice once that the voltage meter was jumping when I use them, however, it hasn't done that again. The voltage meter only shows 8, but when you press the brake you can see it dip slightly.

Here's what I've done so far:
Checked, tightened, and cleaned the grounds that I could find. They appear to be grounded.
Put in a new taillight because one had some faulty wiring at the bulb.
Cut out a trailer wiring harness that I thought was the culprit and rewired the taillights correctly.
Disconnected the headlights from the dimmer switch. The dimmer switch sticks a bit, so I thought it could be the problem.
Had the alternator tested, came back good. The alternator does sometimes make a whirring sound though. The battery tested on the weak side but not bad and the truck always cranks first try.

So I'm currently leaning toward the voltage regulator. But would a faulty reg cause these specific sorts of problems. Could it be there's a ground I've missed? Could the alternator be the issue even though it passed? And why would the brakelights/taillights not light up at all but turn signals work?

I forgot to mention that sometimes, every once in a while, EVERYTHING WORKS PERFECTLY. But there's no way of knowing if it's going to be a good day or a bad day.