Intermitting Dimming of lights on 2001

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On our 2001 silverado, with a 4.8 litre Vortec V-8, the lights on it intermittingly dim and then get brighter again. It does it at a rapid pase, almost like the lights are flashing, but they still stay on. You can see the flashing in the headlights, cabin lights, radio, dash gauges, and ect.  We want to know if it is the alternator or battery, or even something else. please help me figure out what it is. all help is appreciated.

thanks!