Timing Out

Hi!

I made a simple username checker application, in which i provide a list of usernames, and it checks if they exist on a website

First few valid users go okay, but after that, checker keeps timing out, and if i remove the username on which it started timing out, it will do normal till next error
this is the code i got so far:

Code:

 private bool CheckURL(string url)
        {

            CheckForIllegalCrossThreadCalls = false;

            Application.DoEvents();

            Uri urlCheck = new Uri(url);

            WebRequest request = WebRequest.Create(urlCheck);
            request.Timeout = 1500;

            WebResponse response;
            try
            {
                //get URL web response
                response = request.GetResponse();
            }
            catch (Exception)
            {

                return false; //url does not exist
            }

            string responseURL = response.ResponseUri.ToString();


            Application.DoEvents();
            if (string.Compare(responseURL, urlCheck.ToString(), true) != 0) //it was redirected, check to see if redirected to error page
                return !(responseURL.IndexOf("404.php") > -1 ||
                       responseURL.IndexOf("404.htm") > -1);
            else
                return true; //everything okay
        }

        public void checker()
        {

StreamWriter sw;
            CheckForIllegalCrossThreadCalls = false;
            foreach (String s in listBox1.Items)
            {

                    label1.Text = s;
                    if (CheckURL("Auth Page" + s + ".EXTENSION"))
                    {


                        sw = File.AppendText(Application.StartupPath + "/valid.txt");
                        sw.WriteLine(s);
                        sw.Flush();
                        sw.Close();

                        listBox2.Items.Add(s);

                }

            }
        }

I dont have a clue what this could be, so any help would be appreciated

Thank you in advance!