(In reply to comment #5)

I forgot to mention...

The X server runs fine, with no GPU spew in dmesg.  I can use the web browser
and other programs which are not very challenging for the GPU.  DOSBox uses
some OpenGL, and it runs fine; prboom-plus uses a bit more OpenGL, and it runs
OK.  Its when I tested 'torcs' that everything ground to a halt.  After
navigating the torcs menus to start a game, the screen goes black.  One time,
the screen came back for a moment -- the FPS indicator showed 0.2 frames/sec --
before blacking out again.  I was able to get to VT1 with Ctrl-Alt-F1 and kill
torcs, and X was running OK again with I attempted to go back to it with
Alt-F7.