1999 Grand Am L4 Power loss and hesitation

I have a 1999 Pontiac Grand Am that my daughter took to college. Lately, there have been problems with surging and hesitation/misfiring. It started happening at highway speeds after the car was driven for an hour or so. She would try to accelerate and the car would misfire and surge. The service engine light would come on. After the engine was cool again, it would run ok in regular traffic and the light went off. Now the light is on constantly and the car is running horrible under acceleration in any circumstance. I suspected the fuel pump, but have not attempted to replace it yet. I also am wondering about the fuel filter and injectors or some sensor or the coil housing. The plugs were changed about 6,000 miles ago and the oil is chanced regularly. I have also replaced the O2 sensor located in the lower area of the exhaust system by the firewall. I understand there are 2 of them, is this a potential problem as well? I don't want to chase ghosts and have little experience with this vehicle. I have spoken to a mechanic, but have gotten several possible causes, mostly what I suspected to begin with. Where would you suggest I begin dealing with this problem? She is going back to school 200 miles away in a couple weeks, and I need to get this fixed. Thanks for any help you can offer.[/B]