My friends car was doing the same thing. He had the alternator check by Autozone (cause that's where it came from), they said it tested good. Took the car to the shop, and come to find out the alternator was bad. Try swapping out the altenator for another one to see if it's the real problem. But good luck with Autozone, he had to fight with the manager to swap it out, but was well worth it.