Moving object show jerks in middle of Image.

Hi, I am working on 2D car game in Unity 4.3, draw long road and place car and camera (behind car) onto road. When car reach at the middle of road image then it jerks very clearly, I also changed image of road and place another road but still show me one jerks almost middle of road, I don't know why this happened. I stuck over here. Kindly see my below code and tell me where I am doing wrong and also suggested me your experience. Thanks in advance.

float distanceFromPlayerCar =0.0f; float subtractor=0.0f; public Camera playerCarCamera; void Start () { distanceFromPlayerCar = transform.position.z; subtractor=0.3f; }

 void FixedUpdate() {
     distanceFromPlayerCar = distanceFromPlayerCar + subtractor; 
     transform.position = new Vector3(transform.position.x,transform.position.y,distanceFromPlayerCar);
     playerCarCamera.transform.position = new Vector3(playerCarCamera.transform.position.x,playerCarCamera.transform.position.y,distanceFromPlayerCar-2);

} This code script is attached to player car Object.