Problem with login system

[eluser]Unknown[/eluser]
Hi, i'm making a online store and i have little problem with login system. About 3% from all users can't login in to the website.
This is my code.

Code:

public function login()
{
       if(isset($_POST['submit']))
       {
           $email = htmlspecialchars(mysql_real_escape_string($this->input->post("email")));
           $pass = htmlspecialchars(mysql_real_escape_string($this->input->post("pass")));
           $rand1 = htmlspecialchars(mysql_real_escape_string($this->input->post("rand1")));
   $rand2 = htmlspecialchars(mysql_real_escape_string($this->input->post("rand2")));
      $answer = htmlspecialchars(mysql_real_escape_string($this->input->post("answer")));
      $salt = "^^^&&********%^&%^&%^&^&*(&()dfgfdgdfgdko5456dsffds6fsd5fs4d4fsd)";    
      $remember = htmlspecialchars(mysql_real_escape_string($this->input->post("remember")));
   $date = date("Y-m-d h:i:s");
  
   $password = md5(sha1(md5(sha1($pass.$salt.$pass)))); $date = date("Y-m-d h:i:s"); $url = base_url("panel");
  
   $check_user = $this->db->query("SELECT * FROM user_info WHERE user_email='$email' AND user_pass='$password'");
  
   if(($rand1 + $rand2) == $answer)
   {
   if($check_user->num_rows() == 1)
   {
    
      $auth_key = md5(sha1(md5(sha1($email.$date.$salt))));
      
      if($remember == 1)
      {
      setcookie("auth", $auth_key, time()+3600*24*60, "/");
      }
               elseif($remember != 1)
       {
        if(!isset($_SESSION)){ session_start(); }
       $_SESSION['auth'] = $auth_key;
      }
      
      $update_key = $this->db->query("UPDATE user_info SET user_auth='$auth_key',user_activity='$date' WHERE user_email='$email' AND user_pass='$password'");
      
      header("Location: $url");
  
   }
   else
   {
    $this->message = $this->ajax_error("Въведените от вас данни са грешни !<br /><br ?&gt; Моля опитайте пак !","1");
   }
   }else{ $this->message = $this->ajax_error("Въведеният от вас отговор на въпроса е грешен !<br /><br ?&gt; Моля опитайте пак !","1"); }
       }
   }