stutter when going next a cube

I’m making my first game where a cube goes through a level of block having to doge them based off cube field when i go the first few blocks it stutters slowing down then speeds up like a fps drop. so it like goes slow then goes fast. i don’t know is its my code heres my movement code and below my collision code which doesn’t work.

using UnityEngine;

public class PlayerMovement : MonoBehaviour
{
public Rigidbody rb;
public float forwardforce = 750f;
public float sideways = 750f;
// Use this for initialization
void Start()
{
Debug.Log(“hello”);
}

// Update is called once per frame
void Update()
{
    rb.AddForce(0, 0, forwardforce * Time.deltaTime);

    if (Input.GetKey("d"))
    {

        rb.AddForce(sideways * Time.deltaTime, 0, 0);
    }
    if (Input.GetKey("a"))
    {

        rb.AddForce(-sideways * Time.deltaTime, 0, 0);
    }

}

}

collision code with does’nt work

using UnityEngine;

public class PlayerCollision : MonoBehaviour
{

public PlayerMovement movement;    

void OnCollisionEnter(Collision collisionInfo)
{
    if (collisionInfo.collider.tag == "obsticale")
    {
        movement.enabled = false ;   
    }
}

}

@eses what bit are you talking about exactly