What he said, or use a gm alternator. But your problem doesn't really sound like a bad alt, I don't think. Sounds like your headlight switch is going bad or is corroded. It's be worth taking a look at that.
How would a bad headlight switch intermittently activate the battery and brake dash warning lights? headlights work just fine.
I have read that there is a relay mounted behind the glove box that somehow activated the battery and brake warning lights and when it goes bad they can come on intermittently.
