HTTP GET using digest authentication windows server 2003 sp1

Question

 Hello folks, the below code is working fine on my  local machine and also worked fine for 2months on the server. since a few days, the code on the server is NOT  WORKING AFTER 45 MINUTES FROM THE START OF THE TIMER. im seeing this issue since 2 weeks, the same code for  other methods , which use webservices is working fine. am i doing correct with http get?

can someone please let me know waht the issue could be. thanks in advance.

Private dtmA_SentLast As DateTime
initiaize:
dtmA_SentLast = Date.Now.AddHours(-1) when the timer is first started...

Dim tsA As TimeSpan
 tsA = Date.Now.Subtract(dtmA_Sentlast)
        If tsA .Minutes >=5 Then   ----after 45 minutes this is stopped and not  getting data
            If blnSentA = False Then
                ID = GetHttpData()
                dtmA_Sentlast = dtmNow
                blnSentA = True
            End If
        Else
            blnSentA = False
        End If
---GetHttpData() function:

       Dim objProxy As WebProxy
        Dim strResultXML As String
        Dim policy As HttpRequestCachePolicy
        Dim myHttpWebResponse As HttpWebResponse
        Dim myHttpWebRequest As HttpWebRequest
        Try
          
            objProxy = New WebProxy(ProxyIP, ProxyPort)
            objProxy.BypassProxyOnLocal = True

            If ProxyAuthRequired = True Then _
            objProxy.Credentials = New NetworkCredential(ProxyUsername, ProxyPassword)
            '''''
            policy = New HttpRequestCachePolicy(HttpCacheAgeControl.MaxAge, TimeSpan.FromMinutes(5)) --data gets updated every 5 minutes
            myHttpWebRequest = CType(HttpWebRequest.Create(pstrUri), HttpWebRequest)
            myHttpWebRequest.Credentials = New NetworkCredential(pstrUsername, pstrPassword)
            myHttpWebRequest.CachePolicy = policy
            myHttpWebRequest.Method = "GET"
            myHttpWebResponse = CType(myHttpWebRequest.GetResponse(), HttpWebResponse)
          
            Using sr As StreamReader = New StreamReader(myHttpWebResponse.GetResponseStream, System.Text.Encoding.Default)
                strResultXML = sr.ReadToEnd().Trim().ToString()
                myHttpWebResponse.Close()
            End Using
           
        Finally
            '
        End Tryv