I have a 2000 Celica that intermittently misfires when cruising at 60 km/h. Now, this doesn't happen all the time, but when it does, it feels as if the car is heaving. When this suddenly occurs, it pushes you forward slightly as the car slows then the car simply picks up speed again. On occasions I have also spotted the taco needle flicker as this misfire occurs. I had a lag problem repaired recently in which the mechanic replaced the spark plugs, cleaned the injections and cleaned the throttle body. The lag/hesitation is repaired but this new annoying problem has popped up. Any ideas?