ZJ not charging SOMETIMES, is it the alternator or the ECM (computer) ?

Symptoms:
At start-up all is normal, amp gauge at 14V, no dash warnings lit up.
After driving for an hour or so, amp gauge starts dropping, when it gets to 10V then "Check Engine" light comes on.

Happened first last week on Hwy about 4 hours from home. I was sleeping, wife was driving, didn't notice engine light was on. Woke me up when engine started misfiring. I saw Check Engine light was on, and amp gauge was all the way down.

Was Sunday late, nothing open. Got a room. Next morning gauge was normal, so got on the road again. After about 3 hours, same problem, but made it home.

Am amateur shade tree mechanic. I want to know if this is an alternator problem or an ECM problem. Both are original to the vehicle, which I bought new in May of 1992. ZJ has 191,000 miles (some hard, some soft); 2" lift; 31" BFGs; Borla headers; goose and grouse blood on carpeting. Battery is Sears Diehard from 2008. Had a JET power chip in it for a few years, but I removed it about 5 years ago. This is absolutely, beyond the shadow of a doubt, the best vehicle I have ever owned. We love this piece of scrap.

Couple of questions:
1, From my experience, alternators just go out all at once. Is this always true, or can an alternator be erratic, like described above ?
2. Can the above described behavior be symptoms of an ECM failing ?

Any help would be much appreciated.
Thanks

PS. Hope all had a merry Christmas, and a happy new year.
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ZJ, bought it new in '92. 191,000+ miles; 2" lift; 31" BFG's; K/N air; Hella fogs; Borla header; JET power chip; 130% throttle body.