Civic runs rough

I don't mean to turn this into an automotive blog, but I had a bizarre problem with my 1998 Honda Civic and figured it warranted public documentation.

The car would run rough at 20-30 mph (~17% throttle) when the ambient air temperature was 78°-83°F.  There was the occasional backfire when it was really bad.  There was no "check engine" light or other indication that the computer observed trouble.  Diagnostic showed that the secondary O₂ sensor voltage dropped to 0V when the problem manifested itself.  The ultimate fix was to replace the secondary O₂ sensor.

Things I tried, in order:

1. Change plugs and wires
2. Clean throttle body and idle air control valve
3. Change idle air control valve (IACV) ($450)
4. Change the secondary O₂ sensor. ($300)
5. Change the primary O₂ sensor. ($300)

 Now I celebrate the car being repaired!