1998 Oldsmobile Bravada Charging system

Hi, I have a 98 Oldsmobile Bravada with 122K. As I accelerate my cars headlights dim, and when I deccelerate my headlights return to normal. My charge is going from 10-14 as I am driving, but still starts fine. I checked the battery and alternator at Autozone and both were still good. I replaced the belt and this did not help either, an answer would be greatly appreciated.