Thanks for the help with my smooth-running but down on power 244. As several suspected, the cat had failed utterly, and chunks had lodged in the muffler , and a large chunk at one pipe bend. My mechanic said the O2 sensor appeared quite old and came out tough, and since the cat looked much newer, he thinks the O2 might have had it rich which overheated and/or clogged the cat.

So thanks much for the advice.