Alternator, voltage regulator, or battery?

check the ground on the voltage regulator. Make sure that the paint is scraped off the body under the mounting point. I scrape the paint off the back of the regulator (if its the epoxy encapsulated kind) and scrape the body mounting point too. Larger guage cable is good too, and some people run a ground wire from the alt to the volt reg to the battery to ensure a good ground.