hi there,
I think you have to adjust the throttle trim on your controler. your controller has two settings. one is for the steering servo and one for the throtle servo. try to adjust the throttle servo in a neutral position. it sounds like yout car has the brakes on all the time until you hit on the gas. (when you pull the trigger, the car accelerates; when you push it in the other dirrection, the car brakes)
have fun