Player goes in other direction when moving mouse a long time

Hi everyone! I am making a game and I am working on the player control script. When I move my mouse to the left or right for about 5 seconds, the player will start going left like normal, but after a couple seconds, it will head right, then left, all in one mouse move in 1 direction. Anyways, here's the code:

 // Player movement vars
 public var turnSpeed : float;
 public var walkSpeed : float;
 public var sidewaysWalkSpeed : float;
 public var jumpSpeed : float;
 public var gravity : Vector3;

 private var currentGravity : Vector3;

 function Start() {
    Screen.lockCursor = true;
    gravity = Vector3(0, -1, 0);
    turnSpeed = 4.3;
    walkSpeed = 9;
    sidewaysWalkSpeed = 7;
    jumpSpeed = 0.25;
 }

 function Update() {
   RotatePlayer();
   MovePlayer();
 }

 function RotatePlayer() {
   var turnY : float;

   if(Input.GetAxis("Vertical") > 0) {
     turnY = Input.GetAxis("Mouse X") * turnSpeed;
   } else {
     turnY = -Input.GetAxis("Mouse X") * turnSpeed;
   }

   transform.Rotate(0, turnY, 0);
 }

 function MovePlayer() {
   var character : CharacterController = GetComponent(CharacterController);
  
   var movement : Vector3;
  
   movement = Input.GetAxis("Vertical") * walkSpeed * playerCamera.forward *     Time.deltaTime;
   movement = movement + (Input.GetAxis("Horizontal") * sidewaysWalkSpeed * playerCamera.right * Time.deltaTime);
  
   if(character.isGrounded) {
     currentGravity = Vector3.zero;
  
     if(Input.GetButton("Jump")) {
       currentGravity.y = jumpSpeed;
     }
   } else {
     currentGravity += gravity * Time.deltaTime;
   }
  
   movement += currentGravity;
  
   character.Move(movement);
 }