Most research pointed to a bad display board which I replaced with no improvement. Then I read someone's story about how they replaced all the boards in their microwave, the power board being the last one replaced, and fixing the problem. I took a chance, bought a new power board, and the display problem was solved! Looking carefully at the old power board I could see faint evidence of a burn around some resistors. I also noticed a slight dried residue on the back of the board. I suspect it was from steam produced during normal microwave use condensing and drying on the board over the years. The steam/residue mix eventually shorted the board causing it to fail.
Very thankful for having this forum and reading someone else's story to solve my problem. Hope my story helps others to do the same.