Problem solved, though still unsure what it was.
All of the demos were affected.; deleting & downloading them anew did not fix the problem. The TV manual gave no indication it could not handle 720p. Ultimately I went for the nuclear option, the system restore, then downloaded & ran a couple demos (including the GRAW 2 in which the problem first appeared): problem solved. It would have been more of a pain if I'd yet had time to load the hard drive with a lot of content. I let the HDMI automate display settings again after the restore; 720p is ticked & the GRAW demo runs fine, so that would not appear to have been the issue. Some ghost in the machine, I guess. During this process I took the time to re-position the unit so it can stand vertically and it seems to run with considerably less noise and heat. I want it to be happy and serve me well for some time.
It's been a discomforting experience but it all seems to have worked out. I sincerely appreciate the help.
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