OK, got it. It was more than two keys, it was a whole segment of the keyboard. A design weakness of these keyboards is where the circuit board attaches to the Mylar sheet that the keys touch to register the keystrokes. After repeated dis- and re-assemblies, one of the traces on the connector was worn. A touch-up of all the traces with a #1 pencil solved that.

However (you knew that was coming, didn't you)... now there is another problem. Only in Microsoft Word my space bar puts a non-ascii symbol on my screen instead of a space. This symbol looks like a period raised to mid-line height. This symbol does not print, and it does not appear in any other text application (including this one) that I have tried. It only shows up in MS Word.

Oh, hell, I just Googled for "MS Word puts dots instead of spaces" and got 9,960,000 hits. Apparently I'm not the first person to experience this. Let's see what the problem is.

And the problem is caused by... (drum roll, please) Operator Ineptitude! I like to have my paragraph marks show up, so I clicked on the Show Paragraph Mark icon in the Home ribbon. Unbeknownst to me, that turned on all formatting marks, including those scary dots between the words. So the problem is no more, but I leave this here on the chance that it might help someone else.

BTW... with a little digging into the options, there is a lot of customizing that can be done to the ribbon interface.

tanstaafl.