Issue with my Character controller script and textures?

Hello so i was testing a build of my game and i have noticed that when the graphichs quailiy is anything less than "Good" it makes my charater controller act all weird and starts stuttering here is the code

     var forward : Vector3 = transform.TransformDirection(Vector3.forward);
     var right : Vector3 = transform.TransformDirection(Vector3.right);
     var curSpeed : float = walkSpeed * Input.GetAxis ("Vertical");
     var curSpeed2 : float = walkSpeed * Input.GetAxis ("Horizontal");
     if (Input.GetKey(KeyCode.W) || Input.GetKey(KeyCode.A) || Input.GetKey(KeyCode.S)|| Input.GetKey(KeyCode.D)) {
         controller.SimpleMove(forward * curSpeed);
         controller.SimpleMove(right * curSpeed2);
     }

any ideas whats wrong?