Rigidbody cube randomly stops moving?

I'm trying to move a cube with a rigidbody, on a straight surface which also has a box collider, however there are times when the cube would stop moving in all of a sudden, as though it hit a wall and unless i move it in the opposite direction it won't respond at all, even tho it still prints(""); message to the console. I added physics material, set dynamic and static drag to 0.1, i tried using the planes mesh collider instead of box collider, I adjusted the corners to make sure each side are touching, also tried AddForce(), and switching to different collider options (discreet, continuous etc) nothing seems to be fixing this. Any help is appreciated. I'm using the free mini-gold prefabs from the asset store btw.

 private Rigidbody rb;
 public bool reachedGoal = false;
 Vector3 startPosition;
 Vector3 goalPosition;
 Vector3 dir;
 public GameObject goal;
 public float moveSpeed = 5f;

 // Start is called before the first frame update
 void Start()
 {
     rb = GetComponent<Rigidbody>();
     startPosition = transform.position;
     goalPosition = goal.transform.position;

 }

 // Update is called once per frame
 void FixedUpdate()
 {
     //if up/down arrow is pressed and gameobject hasn't made contact with goal
     //call the movement method
     if (Input.GetKey(KeyCode.UpArrow))
     {
         if (reachedGoal.Equals(false))
         {
             dir = Vector3.forward;
             movePlayer(dir);
         }
     }
     if (Input.GetKey(KeyCode.DownArrow))
     {
         if (reachedGoal.Equals(false))
         {              
             dir = Vector3.back;
             movePlayer(dir);
         }
     }
 }
 //check if player made contact with goal
 private void OnTriggerEnter(Collider other)
 {
     if (other.gameObject.tag.Equals("Finish"))
     {
         reachedGoal = true;           
     }
 }

 //add velocity to rigidbody in the direction * speed
 private void movePlayer(Vector3 direction)
 {
     rb.velocity = direction * moveSpeed;     
 }