Simple Car Script - Physics makes it wierd

I am trying to create a simple car script, it works fine when driving across flat terrain but when i go over a bump of a certain height it causes the car to fly.

I originally manipulated the transform of the object to make it move, but i then tried changing the velocity of the rigidbody in a fixed update. For somereason this caused the car to firstly move in the wrong direction i.e. back instead of forwards and the car wont turn, it just rotates and flips in the continued direction.

Clearly im missing something here when it comes to manipulating rigidbody velocity but i cant figure out what it is.

Rigidbody Factors:

Mass: 1 Drag: 0.01 Angular Drag: 0.01 use gravity: true

 using UnityEngine;
 using System.Collections;
 
 public class HumveeScript : MonoBehaviour {
 
     Transform Car;
 
     //Movement Information
     public float movespeed = 0.0f;
     public float reverseSpeed = 0.0f;
     public float currentSpeed = 0.0f;
 
     //Controls
     public KeyCode Accelerate = KeyCode.W;
     public KeyCode Decelerate = KeyCode.S;
     public KeyCode SteerLeft = KeyCode.A;
     public KeyCode SteerRight = KeyCode.D;
 
     public bool hasADriver = false;
 
 
     CharacterController controller;
 
     // Use this for initialization
     void Start () 
     {
         Car = transform;
 
         if (rigidbody)
         {
             rigidbody.constraints = RigidbodyConstraints.FreezeRotationZ | RigidbodyConstraints.FreezeRotationY;
         }
     }
 
     void FixedUpdate()
     {
         if(movespeed > 0)
         {
             rigidbody.velocity = Vector3.forward * movespeed;
             
             //Car.transform.Translate(Vector3.forward * Time.deltaTime * movespeed);
 
         }
         else if(movespeed < 0)
         {
             rigidbody.velocity = Vector3.back * reverseSpeed;
             
             //Car.transform.Translate(Vector3.back * Time.deltaTime * reverseSpeed);
         }
 
     }
 
     // Update is called once per frame
     void Update () 
     {
         if(Input.GetKey(Accelerate) && hasADriver == true)
         {
             if(movespeed < 0 && reverseSpeed == 0)
             {
                 movespeed += 1;
             }
             else if(movespeed >= 0 && movespeed < 20)
             {
                 movespeed += 0.5f;
             }
             else if(movespeed >= 20 && movespeed < 40)
             {
                 movespeed += 1;
             }
 
 
             if(reverseSpeed == 0 && movespeed < 0)
             {
                 movespeed = 0;
             }
         }
         else
         {
             if(movespeed > 0 && movespeed < 50)
             {
                 movespeed -= 1;
             }
         }
 
         if(Input.GetKey(Decelerate) && hasADriver == true)
         {
             if(movespeed > -50)
             {
                 movespeed -= 1;
             }
 
             if(reverseSpeed < 10 && movespeed <= 0)
             {
                 reverseSpeed += 1;
             }
         }
         else
         {
             if(reverseSpeed > 0)
             {
                 reverseSpeed -= 1;
             }
 
             if(movespeed < 0)
             {
                 movespeed = 0;
             }
 
         }
 
         if (Input.GetKey(SteerLeft))
         {
             if (movespeed < 0)
             {
                 Car.transform.Rotate(Vector3.up, 1.0F);
             }
             else if (movespeed > 0)
             {
                 Car.transform.Rotate(Vector3.down, 1.0F);
                 
             }
         }
         if (Input.GetKey(SteerRight))
         {
             if (movespeed < 0)
             {
                 Car.transform.Rotate(Vector3.down, 1.0F);
             }
             else if (movespeed > 0)
             {
                 Car.transform.Rotate(Vector3.up, 1.0F);
                 
             }
         }
     
     }
 
     void OnCollisionEnter ()
     {
         if(movespeed > 0)
         {
             movespeed = movespeed / 2;
         }
         else if(reverseSpeed > 0)
         {
             reverseSpeed = reverseSpeed / 2;
         }
     }
 }