Well, it wasn't a loose terminal...at least not the screw and nut...all were gutentite.
But then I found I can wiggle the negative post, and power returns. Wiggle it again, and I lose power. Then wiggle it again, and power returns. Wiggle it again, and lose power. While without power, don't move anything, and power spontaneously returns.
All this has me wondering if something is amiss inside the battery itself???
Since my battery is 5 or so years old anyhow, I may just replace it with an AGM, and see if symptoms recur. I replaced my Wave Runner batt with an AGM ealier this year. One year warranty, for, I think, about $90 from Advance Auto. Hate to spend $90, but I am due for a battery anyhow.