2000 Ford Focus

Battery light comes on very spuratic for only 5 to 10 seconds at a time. Has new belt, plugs, wires, and alternator was removed and tested at two differant parts stores and passed test. I noticed that my headlights will dim breifly and brigthen back to normal sometimes while driving. The battery was also tested and is good. The battery light can cone on 5 times within 2 miles and not come on again for 2 weeks. The care is driven on a daily basis. Please help