DIC "battery not charging"

2004 Trailblazer EXT 4.2. Started truck, battery light was on and DIC said "battery not charging". I assumed the alternator had failed. Once I got a few miles down the road the light went off and the volts climbed back to 14. Same thing happened once I shut off the truck and restarted. I removed the alternator and had it bench tested a half dozen times, passed every time. Thought I may have a dead cell in the battery so I replaced it along with the serpentine belt. Once all was back together I got the same thing, except it only lasted for maybe 15 - 20 seconds.

Any thoughs guys???

Day two: Alternator finally quit for good. Replaced and all seems well.