The big brown truck delivered the freshly-manufactured clicky keyboard with a semi-future-proof (in this sense, it’s only the future if I live to see it) connector, and the true-blue IBM Model M ‘board with a semi-hemi modern PS/2 connector. I tried them out without hooking them up to the computer. The USB keyboard felt at least as good as the IBM one, so I swapped it for the Lexmark keyboard that had the strange layout and was missing the number pad.
I worked away happily for almost three weeks. I thought I was finished with keyboard flakiness. Last night I was typing away when I noticed that the computer was ignoring me. A reboot fixed it. I am now forced to consider that it is the OS after all, and therefore, given the frequency of the occurrence of the error, essentially unfixable. It is extremely unlikely that it could be a hardware problem, since the old keyboard used the PS/2 connection and the new one uses USB.
I also have discarded what was in all likelihood a perfectly good keyboard.
Computers: can’t live with ’em, can’t live without ’em.