Problem solved.
Installed new OEM Bosch Premium Sensor (13314) and all is well -- engine is glass-smooth and dead silent at idle, and not straining on acceleration, even when hot with the fan clutch engaged.
WARNING: If you see an auction for a low-priced, no-name replacement O2 sensor on ebay,
Don't Buy It. It would look something like this:
http://cgi.ebay.com/ebaymotor/ws/eBayISAPI.dll?ViewItem&item=390257576137
(I will neither confirm or deny that is where I got my failed sensor.)
The eBay sensor is promoted as Bosch, the photo is of a Bosch sensor, and Bosch is mentioned in the listing, but it is definitely not a Bosch part. Compared side by side, the eBay part was at least 1/3 smaller in diameter than OEM, and fit in the Bosch box with plenty of vertical and horizontal room to spare, whereas the Bosch sensor was jammed tight into its box. (The Bosch box could easily hold 2 of those eBay sensors.) The eBay sensor had a black cable, rather than the Bosch blue shown in the listing, and did not have the rubber grommet.
The eBay sensor may have been functional but clearly did not have the working capacity of the OEM. (Translation: It is junk.)
Once again: Lesson learned.