Engine light when passing on highway, O2 code doesn't repeat all the time

I'm getting to my wits end here, I have an 02 alero, with just over 60,000km on it, 2nd gear around 5000 rpm usually when passing on the highway, the check engine light will come on, from that point the car lacks power and gets poor fuel mileage, it's been in the shop repeatedly and they just say it was an O2 Code and they can't reproduce it, clear the code and the car is fine for a week, then back comes on the light. I've already changed the fuel filter, air filter is good, transmission filter was replaced when a faulty pressure control selenoid was repaired. Does anyone know what might be causing this??
Thanks

It is possible that the O2 code may be a lean code..to give you more info, we need to know what the code was.