A couple of weeks ago I was driving on the interstate when my subie started shaking and the check engine light started flashing. I drove it home and the next day I replaced the spark plugs. The car shakes at idle and at certain rpms, especially at higher speeds. It has also been low on power. Yesterday the car suddenly started really sputtering and it seemed to be receiving insufficient fuel when accelerating. The car only creeps forward and then when it gets to a higher rpm then it suddenly roars up and gets going. I don't know what is wrong, whether it's a fuel problem or an ignition problem. Please help me out if you have any idea what my problem might be.