Well, we have progress of a sort. This morning the screen went black and I was expecting the Win8 equivalent of the BSOD message, but things came back, sort of.
Firefox stopped responding after the failure and never quite recovered until I intentionally shut it down. It was OK after I brought it up again.
The message in the system event log is a 4101 from display with the text: Display driver amdkmdap stopped responding and has successfully recovered.
Two of these occurred in very rapid succession, the first at 10:53:02 and the next at 10:53:06.
I have not rebooted the system since then, it's now 1:25 PM, and there have been no additional incidents. I suspect, though, that "this isn't over."