And then, earlier this e'en, hinoki replaced the power supply in my shiny blue brick with andreal's discarded unit.
Lo, it works again!
Ubuntu's reinstalled; dragon mascot or no, I'm not going to try Kubuntu again until I have a better feel for configuring X the hard way, with config files and command lines and blood sacrifice.
Things are MOSTLY working smoothly at this stage, though Firefox is pulling one of those stupid "You need to install this plug-in/This plug-in is already installed/You need to restart Firefox/You need to install the SAME PLUG-IN" things. It should settle out somehow, though, thinking about it, my grandson's machine has run into the same problem.
Grrr, when stuff that Used To Work refuses to work, that means there's probably some sneaky little problem hiding somewhere.
Tomorrow, I have some quality time scheduled with Package Manager. Gnite!