Thanks to all who found/offered a PCI graphics card. My problem has
been solved, even though it hasn't been really understood.
A friend found a card of unknown condition, and I tried it. As
expected, the motherboard graphics became inactive when I booted with
the PCI card. However, the PCI card didn't work either. (It's that
"unknown condition" thing, I think.) After a couple of power cycles,
I removed the PCI card, and, magically, the motherboard graphics began
I don't see anything new in the X config, so I can only think that the
system reset the motherboard graphics card back to a functional
state. If anyone has a better theory, I'd like to hear it.
Thanks to all who responded with an offer of a graphics card.