Unity/C#FPS控制器出现奇怪故障

我有一个奇怪的小故障，当我从一个角落走下来时（我没有按跳跃键），玩家会以非常快的速度摔倒。如果我跳下去，那么一切都会正常。（它的Quill18FPS控制器，我从那里学到了，所以这就是为什么我不使用内置控制器的原因）

using UnityEngine;
using System.Collections;    
[RequireComponent (typeof(CharacterController))]
public class FirstPersonController : MonoBehaviour
{
    public float movementSpeed = 5.0f;
    public float mouseSensitivity = 5.0f;
    public float jumpSpeed = 20.0f;
    float verticalRotation = 0;
    public float upDownRange = 60.0f;
    float verticalVelocity = 0;
    CharacterController characterController;
    // Use this for initialization
    void Start()
    {
//     Screen.lockCursor = true;
       characterController = GetComponent<CharacterController>();
    }
    // Update is called once per frame
    void Update() 
    {
       // Rotation
       float rotLeftRight = Input.GetAxis("Mouse X") * mouseSensitivity;
       transform.Rotate(0, rotLeftRight, 0);
       verticalRotation -= Input.GetAxis("Mouse Y") * mouseSensitivity;
       verticalRotation = Mathf.Clamp(verticalRotation, -upDownRange, upDownRange);
       Camera.main.transform.localRotation = Quaternion.Euler(verticalRotation, 0, 0);
       // Movement
       float forwardSpeed = Input.GetAxis("Vertical") * movementSpeed;
       float sideSpeed = Input.GetAxis("Horizontal") * movementSpeed;
       verticalVelocity += Physics.gravity.y * Time.deltaTime;
       if (characterController.isGrounded && Input.GetButton("Jump"))
       {
          verticalVelocity = jumpSpeed;
       }
       Vector3 speed = new Vector3(sideSpeed, verticalVelocity, forwardSpeed ;
       speed = transform.rotation * speed;
       characterController.Move(speed * Time.deltaTime);
    }
}