2005 Chevrolet Prizm Check Engine light

After one day idle condition when car starts it shows check engine light and after two to three re-start it goes off. Also some time during driving, car suddenly gives little knock and engine stall for a moment comes down from 100 to 80 Km speed and re-gains normal conditions and speed and Engine check light comes up which remain till next stop and again after 2 to 3 re-start it goes. I have changed Air Filter, Fuel Filter and Spark plugs but problem remains same and re-appear.