The problem was the thermostat. It looked to be in excellent shape, but did not open at all, even at 212°F. Odd that it would so suddenly go completely blooie.
With some trepidation I bought a 'Murray+' brand replacement (made in Israel) from Kragen for $6.99, plus $1.29 for an O-ring. Working OK so far.
Anyway, thanks again for all the responses.