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:
- Change plugs and wires
- Clean throttle body and idle air control valve
- Change idle air control valve (IACV) ($450)
- Change the secondary O2 sensor. ($300)
- Change the primary O2 sensor. ($300)