2000 Ford Taurus Electrical

My battery light in my car has been blinking for about 6 months now. I got a new battery and my dad put a new alternator in it 4 years ago. It doesn't do it all the time, just when I go long distances and get up to speeds of over 55. Sometimes it doesn't do it at all. When the battery light comes on, the lights dim and then come back on and dim and then come back on again, over and over and over. My car never stops or acts like it is going to die or anything, just the lights dim. Help me please.