While troubleshooting low & uneven idle or complete stalling, I found that intermittent Throttle Position Sensor will mess with your tests. Throttle Position Sensor are just a rheostat and as they wear the voltage out varies. At idle, RPM's will vary 200 rpm or so (even stall). If you connect the sensor output lead & return/ground lead to a digital VOM, you should have a constant resistance (example-20k ohms) If the Throttle Position Sensor is bad the reading will vary- say 19k to 22k ohms as you watch your meter over a period of time. Try spraying contact cleaner into the Throttle Position Sensor- (throttle body side) if your readings change or if you reconnect & run the engine and it improves or gets worse, REPLACE it. The contact cleaner may correct your problem- for a short time. The fact is the windings inside are worn and nothing can fix that. And of course PO123 error is a sure bet it's bad.
I see people recommend replacing plugs, wires & cap or cleaning the throttle body for uneven idle when many times it's just a bad Throttle Position Sensor. I have even misdiagnosed an Idle Air Control Valve for not idling when it was the Throttle Position Sensor.
Hope this helps.

 

So true.