Rigidbody falling not according to physics, only when script is applied.

Hi,

I've read about this topic for the past 3 hours in an attempt to fix it without asking for help, but I just can't find a solution.

The physics in my scene work perfectly for the primitives I have in there, except for my player. The second I add the playerController script to any asset with a Rigidbody on it they stop being affected by physics the same way.

I've tried upping the gravity, changing the scale of the object, and increasing forces artificially (though my current novice skills fail me). Yet here I am still dumbfounded.

If I import a cube, it falls perfectly fine. I attach my script (basic left, right, sprint and jump functions) and it suddenly acts really light and floaty.

Can anybody help me please?

{

 public float walkSpeed;
 public float runSpeed;

 //JUMP VARS
 bool playerGrounded = false; //Is the player on the ground?
 Collider[] groundCollisions;  //??
 float groundCheckRadius = 0.2f; //
 public LayerMask groundLayer;   //Set in Unity, Layer for ground
 public Transform groundCheck;   //Set in Unity, OBJECT for ground detection
 public float jumpHeight;        //jump height
 bool running;

 Rigidbody playerRB;

 // Use this for initialization
 void Start()
 {
     playerRB = GetComponent<Rigidbody>();
 }   
 
 void FixedUpdate () {
     
     //Movement left and right
     running = false;

     float move = Input.GetAxis("Horizontal");           //Assigns A and D to move +1 or -1

     float sprint = Input.GetAxis("Fire3");              //Shift
             

     if ((move > 0 || move < 0) && (sprint > 0 && playerGrounded)) //If A or D && Shift && Grounded
     {
         
         playerRB.velocity = new Vector3(move * runSpeed, 0, 0);     //This one triggers if moving and holding shift whilst moving
     }
     else
     {
         playerRB.velocity = new Vector3(move * walkSpeed, 0, 0);    //This one allows movement
     }
     //----
    

     //JUMPING

     //Set the ground check collider
     groundCollisions = Physics.OverlapSphere(groundCheck.position, groundCheckRadius, groundLayer);

     //Is the player on the ground?
     if (groundCollisions.Length > 0) //Array.Length - A collision will add 1 or more to the array
     {
         playerGrounded = true;
     }
     else
     {
         playerGrounded = false;
     }

     if (playerGrounded && Input.GetAxis("Jump") > 0)
     {
         playerGrounded = false;
         playerRB.velocity = new Vector3(playerRB.velocity.x, 0, 0); //Sets the Y velocity (up/down) to 0 for consistency
         playerRB.AddForce(0, jumpHeight, 0, ForceMode.Impulse);
     }
    

  } //end of fixed update

}

Any help you can provide will be greatly appreciated, thanks for your time, and I apologise if this question has already been answered, it's now 3am and I've been sat here trying not to pull my own eyes out since midnight. Thanks again.