John is right about the diode going bad. I would wonder if worn brushes could mess up the voltage regulator function. A regulator/brush assembly is less bux than another alternator, except from a boneyard. Neither item is usually returnable as electrical items usually aren't allowed. If you get a regulator/brush assembly and it isn't the culprit, then you have a good spare unit.

One more thing, I experienced the flickering one time after an extremely rough rocky road trip. Turned out the battery was so jarred around that some of the coating on the plates was knocked loose and had fallen to the bottom and shorted out the plates. The voltage regulator was trying to charge, but the battery was too much of a short circuit so the regulator was "hunting" up and down and up and down. Have the battery checked first, it's likely the lowest cost thing to try.

Good Luck,

Bob

:>)