[Daves55Sedan]

I've got good clean tight battery-to-engine, engine-to-frame and engine-to-body grounds. That's not the problem.
Both times this happened, were immediately after going up a little incline (parking lot entrance and my driveway). So I did wonder if sediment in the battery might be shorting out the bottom plates, but battery is exactly 3 years old as of last month (Aug).
The positive battery cable had one of those battery post clamps that has the cable clamped onto it (rather than the wire molded into the clamp). I jerked it around some and all the power came back on, but that doesn't necessarily mean anything. Just for the heck of it, I took that entire cable and clamp assembly off and tossed it. I put an old used battery cable that had a good molded-on clamp instead (after cleaning both ends and both terminal posts at battery and solenoid).
However, I am still going to crawl under the dash tomorrow AM and check the big yellow wire connections at the starter switch and headlight switch for looseness or corrosion. I will let you know what I find.