I recently changed all my spark plugs when i got a p303 code. The problem was fixed then came back. When i replaced cylinder 3 i noticed the spark plug had a imprint from the electricity arcing on the spark plug instead of going to the tip. I can't seem to find the problem when I idle the car seems to drop rpms all of sudden and shudder shaking the whole car then the rpms will jump back up. I noticed the car also shudders when driving in first and under hard acceleration it seems to have a hard time. I recently got the o2sensor replaced because my cars mpg dropped to 10, after the replacement of the o2 sensor the mpg is back up to 24. any ideas what could be wrong?