Tonight I noticed that things are, again, slowly growing darker. Seems this monitor just has a higher brightness & contrast settings than the previous one (so it covered up the problem). I think I've managed to ferret up another video card that'll fit with some reworking -- The original video card was/is a video + audio card that plugs in pretty far in the box. I added a second video card above it and had been using that one quite some time ago. A newer spare computer has separate video and audio cards, but the video plugs in pretty far back, so it looks like I'll have to remove the old V&A one I'd been using just for the audio, so I can fit the "new" video one in, and then pop in the "new" audio one above it.
Just in case anyone wonders why I might vanish off for a day or so, it's probably due to my having to futz around with that. I'd do it tonight, but I'm getting a bit too sleepy to want to deal with any complications (and it's easier to just check my E-Mail through a monitor darkly tomorrow morning, then return to fiddling pre-work).
ETA: No way the "new" video card'll fit, so I'll be digging around for that. The old V&A one came out like a charm (so the system starts faster, since it doesn't check for the video hardware that it won't use, then clean the slate and look for the back-up), but it'll take some E-Digging to grab a driver for the "new" audio card (and I'll have to yank it out before then, since the model-info is on the underside of the card).