Don't need to ponder this anymore: last week at the end of a trip across Florida, a big front vibration quickly emerged. Upon inspection, the tires were significantly cupped on the outer tread. When removed, a pronounced bulge on the inside with distortion in the tread at that location seemed to indicate a separation in a belt. So it appears that the problems were due to a structural issue in the tires.
Like Groover advised, I'll run 10-15 psi more than what the chart lists.
FYI: McGee tire in Daytona installed Firestone ST230 tires for $667 out the door. Good tech and good service.
Thank you all for your interest and good comments