Pontiac Grand AM problem

Hi, Two weeks back check engine sign came...mechanic replaced the oxygen sensor and serviced the car.. A week back, engine sign began to show again, and the engine began to lose its power beyond 50mph and the ETS off sign would not go. Was okay for while when the check engine sign would not come. For the last four days, it has gone to the worst. All the lights went out and then it didnt start. I thought because of cold. The next day, it started and I drove it to mechanic who ran it in computer and found no problem. I came back home, and now the car starts with much problem, but the engine shakes as if it is gonna burst. And when I shift to drive the car shakes more and stops. What is it? I am suspecting the starter, or bad fuel.. Help me.. Thanks.