HELP!!! Charging issue.

[sacaledus]

Thank you in advance for any and all help. Well I have a 1979 C20 with the 5.7L 350. It runs good but I am encountering a charging issue. New battery, all connections look good, alternators been tested both on and off vehicle and passes. Here's the issue. Everytime I jump on the freeway I get a dip in voltage. Lately it's been into the red. One night it dropped down so far I pulled to the side of the road and let id idle and it jumped up just past 13 then came back down to just below. Put it in gear to take back off and it begins to drop. Got it home and at idle jumped back up to 13 and stayed while idling. Any ideas??