If you haven't already done so, I'd sure check very carefully ALL GROUNDS.

It's not unusual to find the ground cable (negative) to the battery to be
eaten almost copletely at the battery and poor or corroded connections at the
other end aren't unusual.

Don't assume anything, check each cable connection in the electrical system,
clean any connections that look at all questionable and tighten down well
after cleaning.

I suppose that it's also possible that there's a fusible link that has become
intermittent.  They act like a one-time fuse to protect the electrical system
if there's a short or heavy current draw.

You probably need to get somebody who KNOWS something about automotive
electricity/electronics to help troubleshoot.  Many mechanics don't have a
clue if it's not a common problem.

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Were there any bright lights in the sky which you could not account for?