Help With Quaternion.LookRotation

I'm making a fairly basic 3D platformer, and trying to have my character look in the direction they are facing. When I try out the game with the line disabled, the script works fine (allows the character to move and jump). However, when I enable the line with Quaternion.LookDirection and try moving, the character rotates absurdly fast and cannot be controlled. I have a feeling I'm doing this wrong but can't find anything on how exactly I'm supposed to be using it.

 using UnityEngine;
 using System.Collections;
 
 public class Movement : MonoBehaviour {
 
     public float speed = 6.0F;
     public float jumpSpeed = 8.0F;
     public float gravity = 20.0F;
     private float overAllSpeed;
     private Vector3 moveDirection = Vector3.zero;
     void FixedUpdate() {
         CharacterController controller = GetComponent<CharacterController>();
         if (controller.isGrounded) {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0.0f, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             if (Input.GetButton("Jump"))    {
                 moveDirection.y = jumpSpeed;
             }
         
         }
         else if(controller.isGrounded==false)    {
         moveDirection.x = Input.GetAxis("Horizontal");
         moveDirection.z = Input.GetAxis("Vertical");
         moveDirection = transform.TransformDirection(moveDirection);
         moveDirection.x *= speed;
         moveDirection.z *= speed;
         moveDirection.y -= gravity * Time.deltaTime;
         
         }
      
         controller.Move(moveDirection * Time.deltaTime);
         overAllSpeed=Mathf.Abs((moveDirection.x+moveDirection.z)/2);
         Debug.Log(overAllSpeed);
         if (overAllSpeed!=0)
             transform.rotation = Quaternion.LookRotation(moveDirection);
     }
 }