Move player with mouse help

I have this script that moves the object on a plane on the mouse position.

 // Click To Move script
 // Moves the object towards the mouse position on right mouse click
  
 var smooth : float; // Determines how quickly object moves towards position
 var selected : boolean;
 
 private var targetPosition : Vector3;
  
 function Update () {
     if(Input.GetKeyDown(KeyCode.Mouse1) && selected)
     {
         var playerPlane = new Plane(Vector3.up, transform.position);
         var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         var hitdist = 0.0;
  
         if (playerPlane.Raycast (ray, hitdist)) {
             var targetPoint = ray.GetPoint(hitdist);
             targetPosition = ray.GetPoint(hitdist);
             var targetRotation = Quaternion.LookRotation(targetPoint - transform.position);
             transform.rotation = targetRotation;
         }
     }
  
     transform.position = Vector3.Lerp (transform.position, targetPosition, Time.deltaTime * smooth);
 }
 
 function OnMouseStay() {
     if(Input.GetKeyDown(KeyCode.Mouse0))
     {
         selected = true;
     }
 }

but there are some bugs. firstly the object falls through the ground slowly, and it moves in a long distance very fast and on short distances very slow. How can I set the speed to be the same always??