Sno's advice was very useful; I found that three of my USB ports had the power-saving function on. Now they're off (for the present). But that didn't solve the problem.
Drano put his finger on it. The oldman had the HDMI plugged into the computer's port, not the video card port. I blame the dog for making me pull it out in the first place.
Thanks, everyone, I don't know how many months it would have taken me to discover such a simple error on my own.
- oldman