Thanks, Olaf.
After looking through all the settings for shooting (and anything else I thought might be a variable,) I finally just performed a full reset to the default shooting menu. That corrected the problem. I still have no idea why the pictures taken before the reset were so degraded. In simply looking at them through the viewfinder, they reminded me of high ISO shots 20 years ago.
The mystery remains unsolved. But my love of the camera is back.