at high rpms the belt could be slipn on the pulley. Is there any rubber resedue on the face of the alternator? But most likely the alternator is going out due like kneedownnate said and take it in to have it tested they can test it on the vehicle as well as off the vehicle. With it off atleast you can know one way or the other, but with the whole charging system checked at like kragen or autozone it could be regulator, battery, or a multitude of things
