Ford Taurus 1999 Misfire Code P1309

This car has a V6 engine (SE model), and over 120K miles. CAM Shaft was changed along with the spark plugs/wires, radiator sensors. The engine light came on again and the same code was generated. As a last resort, PCM was changed. The engine light disappeared and the next day after driving the car for over 1 hour, I took it in for smog test. It passed smog test. Later the same day, the light came on again and this time the code are for misfire (P1309) and bad downstream Oxygen Sensors. I need help.