Something to have a look at is an exhaust leak. ANY leak upsteam of the O2 sensor will fool it into thinking it is lean and richen the engine to compensate. Also make sure the seal for the O2 sensor is properly seated.
It took a long time to get my exhaust leaks fixed but I'm pretty sure I got them all licked. Wouldn't it have to be a good size exhaust leak to fool the o2, it doesn't take a lot of exhaust for the o2 to take readings.
