Dimming lights and intermittent engine cutout when stepping on brakes with lights on

Car has 6month old alternator and battery is about 18months. I had load test done alternator checked ok and battery did not fail but showed moderate recovery. This same thing happened 6months ago and I put new alternator in and also new belt tensioner and everything was good for 6months. I noticed this time when engine is on and I turn on headlights that clock on radio dims and stays dimmed. With car parked and engine running can not duplicate problem, but as soon as you start driving each time I brake the headlights dim for a second and then after driving a few miles engine cuts out but recovers. I looked for loose wires and battery terminals are clean. It acts like a bad alternator, but test showed ok. Maybe a bad switch? Any help would be appreciated. Thank you.