Intermittent battery not charging for about two minutes?

Car starts and runs fine. Then sometimes, following occurs, red battery light on dash, dash voltage drops about 1/2, warning message of battery not charging. Lasts for a minute or two then goes away. Normal voltage at battery but have not been able to check it when above happens. New battery last week, did not correct. Suspect, ground cable, voltage regulator, alternator. Where should I begin? The alternate output in little red box looks rusty so I thought that I should remove battery ground, remove that and wire brush for possible starters.