StrangeCharging issue

[Cape Codder]

I have a 98 1500 with a 4.3. The truck charges fine while idling but but while driving especially under acceleration the gauge drops to Bat voltage. It doesn't happen all the time and it doesn't matter if it is light or hard acceleration. It is very intermittent but happens enough to be a concern. I replaced the alternator thinking it could be something with the internal regulator and it made no difference I just replaced the belt tensioner which was weaker than the new one but still having the problem. The Bat. light also flashes when the gauge drops. If I take my foot off the gas and shift to neutral it goes back up.
Any ideas? The battery hasn't died but I am afraid to go to far from home now. It acts the same no matter how many things I turn on. The truck is a W/T with no power accessarys.