Check your fuel trims. If you notice that they are very far positive it could be your o2 sensor, but it's hard to say otherwise.
If you can log the car I'd check that your car is in closed loop going down the road, and if not you will have to figure out why...
Off the top of my head I'd say o2 sensor, but I HATE throwing parts at a car and hoping that it fixes the problem.
If you want I could check the fuel trims and maybe spend a few minutes logging the car and let you know if there is anything that sticks out.