2003 Mazda Tribute lost power

I have a 2003 Mazda Tribute with the V6. The battery light will occasionally come on, sometimes it just flashes, other it stays on. I had the battery and alternator checked at two different auto parts stores and they both tested fine. I checked it with a voltmeter and even did the old take off the positive terminal to check the alternator trick and in theory, they’re both fine. Yesterday I was in a long drive through window line with my stereo on and the AC on, and the stereo shut off and the car started to lose power as if the battery was dying. My question is, what could cause this? Thanks for any answers I get.