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?

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