This has probably been discussed here before, I searched but didn't see anything...
Recently my '04 Astro LT AWD w/90K has had an intermittent battery light showing and voltage dropping below 14V on the voltmeter and test meter at times. I tested the battery and alternator with the original AC Delco battery being most suspect with the weather getting warmer here and so replaced that with a new Interstate Megatron Plus. In checking and testing the alternator while installing a new belt tensioner/belt and fan clutch and checking grounds and power leads, I came a across a loose and sloppy fitting connector on the alternator. The connection was poor causing the intermittent battery light and other condition so after a bit of fiddling with the connector to tighten it up and make better contact, the problems have gone away. I will still probably replace the alternator soon as I am not fully happy with it's output and replace the suspect connector.
Later...