IsGrounded keeps flickering on and off, then stays off after a few seconds

hey guys, i am quite new to coding games, so i try my best to follow tutorials so that i can learn from them. this code is from brackey and another code related to brackey's code. All I want is to make the isGrounded check box to turn on when it hits a block labeled in the layer "Ground", which does work but then the ground check starts spazzing out and keeps turning off and on again and again, after a few seconds it just stays off. I don't know what I have done but here's the code:

 public class PlayerMovement : MonoBehaviour
 {
     //VARIABLES
     [SerializeField] private float moveSpeed;
     [SerializeField] private float walkSpeed;
     [SerializeField] private float runSpeed;
 
     private Vector3 moveDirection;
     private Vector3 velocity;
 
     [SerializeField] private bool isGrounded;
     [SerializeField] private float groundCheckDistance;
     [SerializeField] private LayerMask groundMask;
     [SerializeField] private float gravity;
 
     [SerializeField] private float jumpHeight;
 
     //REFERENCES
     private CharacterController Controller;
 
     private void Start()
     {
         Controller = GetComponent<CharacterController>();
     }
 
     private void Update()
     {
         Move();
     }
 
     private void Move()
     {
         isGrounded = Physics.CheckSphere(transform.position, groundCheckDistance, groundMask);
 
         if(isGrounded && velocity.y < 0)
         {
             velocity.y = -2f;
         }
 
         float moveZ = Input.GetAxis("Vertical");
 
         moveDirection = new Vector3(0, 0, moveZ);
 
         if(isGrounded)
         {
             if (moveDirection != Vector3.zero && !Input.GetKey(KeyCode.LeftShift))
             {
                 Walk();
             }
             else if (moveDirection != Vector3.zero && Input.GetKey(KeyCode.LeftShift))
             {
                 Run();
             }
             else if (moveDirection == Vector3.zero)
             {
                 Idle();
             }
 
             moveDirection *= moveSpeed;
 
             if (Input.GetKeyDown(KeyCode.Space))
             {
                 Jump();
             }
         }
 
             
 
         Controller.Move(moveDirection * Time.deltaTime);
 
         velocity.y += gravity * Time.deltaTime;
         Controller.Move(velocity * Time.deltaTime);
 
         
     }
 
         private void Idle()
         {
 
         }
 
         private void Walk()
         {
             moveSpeed = walkSpeed;  
         }
 
         private void Run()
         {
             moveSpeed = runSpeed;
         }
 
         private void Jump()
     {
         velocity.y = Mathf.Sqrt(jumpHeight * -2 * gravity);
     }
 }