c# HttpWebRequest多线程获取网页数据，提示操作超时

    c# HttpWebRequest多线程获取网页数据，把网址放到线程池中，大概打开20多个网页后就会提示“操作超时”或者“连接已关闭”，求教如何解决？以下是部分代码：//下面是线程池，把网址作为参数传给线程
            for (int i = 0; i < adr.Count; i++)
            {
                OddsThreadParameters tpNum = new OddsThreadParameters { num = i   ,singleItem=singleBaseData[i]};
                ThreadPool.SetMaxThreads(5, 5);
                ThreadPool.QueueUserWorkItem(new WaitCallback(SingleT), tpNum);
            }
//下面是SingleT函数的主要内容，打开网页，就是这个地方出错
System.Net.ServicePointManager.DefaultConnectionLimit = 100;
            string pagedata_detail = string.Empty;
            for (int reGet = 0; reGet < 3; reGet++)
            {
                HttpWebRequest wrq_detail = HttpWebRequest.Create(adr[tpNum.num]) as HttpWebRequest;
                wrq_detail.Method = "GET";
                wrq_detail.ProtocolVersion = HttpVersion.Version10;
                wrq_detail.KeepAlive = false;
                wrq_detail.Timeout = 20000;
                try
                {
                    pagedata_detail = GetWebDataWithZipJuge(wrq_detail);
                }
                catch (System.Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
            }
//GetWebDataWithZipJuge函数用来返回网页string
 public string GetWebDataWithZipJuge(HttpWebRequest wr)
        {
            //下面方法用来判断网页源码是否压缩，以获取数据
            System.Net.ServicePointManager.DefaultConnectionLimit = 100;
            wr.Method = "GET";
            wr.ProtocolVersion = HttpVersion.Version10;
            wr.KeepAlive = false;
            string data = null;
            using (HttpWebResponse response = (HttpWebResponse)wr.GetResponse())
            {
                if (response.ContentEncoding.ToLower().Contains("gzip"))
                {
                    using (GZipStream stream = new GZipStream(response.GetResponseStream(), CompressionMode.Decompress))
                    {
                        using (StreamReader reader = new StreamReader(stream, Encoding.Default))
                        {
                            data = reader.ReadToEnd();
                            response.Close();
                        }
                    }
                }
                else if (response.ContentEncoding.ToLower().Contains("deflate"))
                {
                    using (DeflateStream stream = new DeflateStream(response.GetResponseStream(), CompressionMode.Decompress))
                    {
                        using (StreamReader reader = new StreamReader(stream, Encoding.Default))
                        {
                            data = reader.ReadToEnd();
                            response.Close();
                        }
                    }
                }
                else
                {
                    using (Stream stream = response.GetResponseStream())
                    {
                        using (StreamReader reader = new StreamReader(stream, Encoding.Default))
                        {
                            data = reader.ReadToEnd();
                            response.Close();
                        }
                    }
                }
            }
            return data;
        }