I'm having a wierd problem using HttpWebRequest/HttpWebResponse when
posting to a third party server via https to process credit card
transactions.  The WebException (SecureChannelFailure) I am getting
occurs during the call to GetResponse()  (see code below) and returns
the message 'The underlying connection was closed: Could not establish
secure channel for SSL/TLS'.  I've seen this error in many other posts
but my situation seems a little different.  Basically, the FIRST time
an end user submits a transaction on our website, the below code gets
executed and when the exception is thrown an error is displayed to the
end user.  If the user hits their back button and then 'submits' again
-- it works fine -- and, will now ALWAYS work fine from that end
user's computer.  This is wierd to me because when the https request
is made from our server to the third party server the actual end
user's environment (machine, browser, etc...) should be out of the
picture (i.e. - not have an affect on the https request/response being
made between the servers) -- right?  But apparently it does, because
it only fails the first time a user tries to submit, after that it
works fine all the time.  Any help on this would be GREATLY
appreciated!!!  Here is my code:

REQUEST/RESPONSE CODE:
    ServicePointManager.CertificatePolicy = new
AcceptAllCertificatePolicy();

    httpRequest = (HttpWebRequest) WebRequest.Create("https://........");
    httpRequest.Method = "POST";
    string sPostData = "some data I setup for the post"
    byte [] postBuffer = System.Text.Encoding.ASCII.GetBytes(sPostData);
    httpRequest.ContentLength = postBuffer.Length;
    httpRequest.ContentType = "application/x-www-form-urlencoded";
    httpRequest.Timeout = 100000; // wait 100 seconds for a response
    httpRequest.AllowAutoRedirect = true;
    Stream requestStream = httpRequest.GetRequestStream();
    requestStream.Write(postBuffer ,0,postBuffer.Length);
    requestStream.Close();

    httpResponse = (HttpWebResponse) httpRequest.GetResponse();
    stream = httpResponse.GetResponseStream();
    streamReader = new StreamReader(stream,System.Text.Encoding.ASCII);
    sResultLine = streamReader.ReadLine();
    while (sResultLine != null)
    {
        // read all lines
    }

    streamReader.Close();
    httpResponse.Close();

REQUEST/RESPONSE CODE:
    public class AcceptAllCertificatePolicy : ICertificatePolicy
    {
        public bool CheckValidationResult(ServicePoint srvPoint,  
            System.Security.Cryptography.X509Certificates.X509Certificate
certificate,
            WebRequest request,
            int certificateProblem)
        {

            // Just accept.
            return true;
        }
    }