Update from last night.
Of course the map changed so this wasn't a real apples to apples compare with the map as a constant.

I found every single video and graphics options and increased them all and re-started the game. Then I set them all down as far as possible and re-started the game.
That seems to have corrected the problem. My frame rate stabilized at 59.

Next step, maybe, when I get over the happiness and elation of being able to play this game again

will be to move them up one at a time and see what happens.