my check engine light is only on when i drive the car, but when i try to scan it, the check engine light doesnt come on when the ignition is in the "on" position so i can't scan it. and if i do scan it, it comes up clear.. the problem my car is having is wierd because it will go for weeks driving perfect (with the check engine light on), but then all of a sudden it might just sputter out, and when i push on the pedal it just sputters more and shakes a little bit. any ideas? and why doesnt my check engine light stay on? is there something wrong with my computer?