vanilla gorilla

Stock car. Anyways, for the first 15-20 miles or so the car runs awesome, after that the battery light start flickering, doesnt do it constantly, it will come on then go off then on and off. I took the alternator off and tested it at my store and it was putting out 15 volts. Car has a great battery in it also, starts the car no problem. I just cant figure it out. When the alternator is bad....it would run like poop the whole time. But this car runs good until it gets hot. But the alternator is good. What else could it be. Isnt there a regulator or something that controls the charging? Any help would be greatly appreciated.