I've seen this before when a diode or two popped in the alternator. Ac voltage causes the flicker. Rare, but happens.
A swap with a know good or remove it and have it tested, though I've seen bad alts pass the typical bench check.
It's also possible to disconnect the field and positive wire from the alt to see if it goes away - problem is that this test doesn't rule out the regulator. Alternator problems can be tough to diagnose.
