Character goes crazy when pressing play

Hey, I m currently making a endless runner game but I bumped into a problem, I dont know how to fix. the problem is that the character goes crazy bounces,bumps into objects... anything is helpful.. thanks.

script:

 using UnityEngine;
 using System.Collections;
 
 public class PlayerMotor : MonoBehaviour 
 {
     private CharacterController controller;
     private Vector3 moveVector;
 
     private float speed = 5.0f;
     private float verticalVelocity = 0.0f;
     private float gravity = 5.0f;
         
     private float animationDuration = 3.0f;
 
     private bool isDead = false;
 
     // Use this for initialization
     void Start () {
         controller = GetComponent<CharacterController> ();
     }
 
     // Update is called once per frame
     void Update () {
 
 
         if (isDead)
             return;
         
         if (Time.time < animationDuration) {
             controller.Move (Vector3.forward * speed * Time.deltaTime);
             return;
         }
 
         moveVector = Vector3.zero;
 
         if (controller.isGrounded) {
             verticalVelocity = -0.5f;
         } else {
             verticalVelocity -= gravity * Time.deltaTime;
         }
 
         // X - Left and Right
         moveVector.x = Input.GetAxisRaw ("Horizontal") * speed;
 
         // Y - Up and Down
         moveVector.y = verticalVelocity;
 
         // Z - Forward and Backward
         moveVector.z = speed;
 
         controller.Move (moveVector * Time.deltaTime);
     }
 
         public void SetSpeed(float modifier)
         {
             speed = 5.0f + modifier;
         }
     // It is beign called every time our capsule hits something
     private void OnControllerColliderHit (ControllerColliderHit hit)
     {
         if (hit.point.z > transform.position.z + controller.radius)
             Death ();
     }
 
     private void Death ()
     {
         isDead = true;
             GetComponent<Score> ().OnDeath ();
     }
 }