When something like your a/c or fan in your engine turns on, that will usually cause it to do that to my car. Its just doing it because power is being drawn from your battery. It happens to me if I leave my car running and the engine starts to get hot, then the fan kicks on and power is drawn from the battery and the lights will dim for a second or so. I would say that you are going through the same deal? It happens at idle to me too.