Stuttering Player Movement

Hey guys, currently when my player is moving he is stuttering. I don't know how I should fix that. Hope that someone has a solution.

Here's my movement script:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class Movement : MonoBehaviour {

//Variables public CharacterController controller; public float gravity = -9.81f; public Transform groundCheck; public float groundDistance = 0.4f; public LayerMask groundMask; public float jumpHeight = 3f; public float speed;

Vector3 velocity; bool isGrounded; private bool canDoubleJump;

 void Start()
 {

 }



 void Update()
 {



   //Gravity & GroundCheck
   isGrounded = Physics.CheckSphere(groundCheck.position, groundDistance, groundMask);

   if(isGrounded && velocity.y < 0)
   {
     velocity.y = -4f;
   }



   //Dash
   if (Input.GetKeyDown(KeyCode.LeftShift))
   {
        speed = 14;
   }

   if (Input.GetKeyUp(KeyCode.LeftShift))
   {
        speed = 7f;
   }




   //Movement
   float x = Input.GetAxisRaw("Horizontal");
   float z = Input.GetAxisRaw("Vertical");

   Vector3 move = transform.right * x + transform.forward * z;

   velocity.y += gravity * Time.deltaTime;
   controller.Move(move * speed * Time.deltaTime);
   controller.Move(velocity * Time.deltaTime);




   //Gravity changing with button
   if(Input.GetKey("2"))
   {
     gravity = -0.2f;
   }

   if(Input.GetKey("1"))
   {
     gravity = -20f;
   }




   //Jump / DoubleJump
   if(isGrounded)
   {
     canDoubleJump = true;
   }

   if(Input.GetButtonDown("Jump"))
   {
     if (isGrounded) {
     velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
   } else {
     if (canDoubleJump == true) {
     velocity.y = Mathf.Sqrt(jumpHeight * -2f * gravity);
     canDoubleJump = false;
     }
   }




 }

} }