My car is a 98 alty, auto tranny. When I stop at a light, sometimes the car stalls. Other times is comes really close to stalling, the rpms dip down below 500 and jump up again. Also when I'm driving at any speed, sometimes the car just jerks back and forth, the rpms also dip down. I've taken it to the dealer but they said no trouble codes have been tripped. Could this be a bad spark plug? Or maybe I gapped the spark plugs too far?