Do you have a older traxxas TQ AM radio? I had one go bad in one direction but the other direction was fine. One way you can trouble shoot this problem (if you dont have any other servos) is to unplug the steering servo and the throttle servo on the RX and switch them. You should be able to controll the throttle with your steering wheel now. If your problem is still happening you will know it's not the servo, your problem will either be in your radio or your RX.