2002 Chevy Monte Carlo Why to the lights blink on/off after

Hello,
Recently while driving my car the voltmeter in the dash started bouncing from high voltage to low voltage causing the battery light to come on intermittantly. I got the car home and turned it off. The front lights as well as the backup lights would click on and off and does so for hours at a time. At times it will start and run like nothing is happening and runs fine. I took it to a auto parts stop and asked them to check the alternator and battery and they said they were fine. The car starts regularly but the voltmeter gage and lights continue to do what they've been doing. My thougt it there is some kind of short somewhere that is cuasing this.