Need help

Have you driven it at night to see if the headlights dim when this happens? Could be a simple loose wire between the alternator and the light or could be the alternator. Electric issues if random are hard to find sometimes. Does it do it all the time or when the road is rough. If you could have someone put a test light on something that gets electric from engine such as cigarette lighter when it was doing it, you could check output. You did not mention it but I am assuming it was not doing it when you took it in for service.