outbound web requests fail when made from IIS sites using shared IP address

• IIS 7.5 / Windows Server 2008 R2
• Multiple IIS sites bound to the same IP address, using host names.
• Inbound traffic to sites working fine.
• Outbound web requests made by the back-end site code fail. Remote site returns 404 (NotFound).
• Verified via a network trace that the traffic is making it to the remove server.
• Same requests work fine if done from a site using a dedicated IP address (i.e. not shared w/ any other sites).

Anyone have any ideas on how to make this work or what could be going wrong?

Network trace on hosting server:

Successful request from site w/ non-shared IP address:

No.     Time            Source                Destination           Protocol Info
   6366 15:54:35.590463 192.168.1.76          173.194.77.121        HTTP     GET /key/value/one/two HTTP/1.1 
   6369 15:54:35.599879 173.194.77.121        192.168.1.76          TCP      http > 55407 [ACK] Seq=1 Ack=110 Win=344 Len=0
   6370 15:54:35.621587 173.194.77.121        192.168.1.76          HTTP     HTTP/1.1 200 OK  (application/json)
   6608 15:54:35.815774 192.168.1.76          173.194.77.121        TCP      55407 > http [ACK] Seq=110 Ack=357 Win=509 Len=0

Failed request from site using a shared IP address:

No.     Time            Source                Destination           Protocol Info
   9720 15:54:39.244192 192.168.1.80          173.194.77.121        HTTP     GET /key/value/one/two HTTP/1.1 
   9760 15:54:39.256958 173.194.77.121        192.168.1.80          TCP      [TCP segment of a reassembled PDU]
   9761 15:54:39.256962 173.194.77.121        192.168.1.80          HTTP     HTTP/1.1 404 Not Found  (text/html)
   9762 15:54:39.257027 192.168.1.80          173.194.77.121        TCP      55438 > http [ACK] Seq=212 Ack=1676 Win=512 Len=0

Code:

public static HttpWebRequest CreateWebRequest(string url, string method = "GET", string referer = null, string contentType = null, int timeout = 100000, string authentication = null, string bindToIpAddress = null, string host = null)
{
    var request = (HttpWebRequest)WebRequest.Create(url);

    if (!string.IsNullOrWhiteSpace(bindToIpAddress))
    {
        IPAddress bindIp;
        if (!IPAddress.TryParse(bindToIpAddress, out bindIp))
        {
            throw new ArgumentException("bindToIpAddress");
        }

        request.ServicePoint.BindIPEndPointDelegate = ((sp, rep, rc) =>
        {
            return new IPEndPoint(bindIp, 0);
        });
    }

    request.Accept = "*/*";
    request.ContentType = contentType;
    request.Referer = referer;
    request.Method = method;
    request.Timeout = timeout;

    if (!string.IsNullOrWhiteSpace(host))
    {
        request.Host = host;
    }

    return request;
}

string GetData()
{
    try
    {
        string result;

        var request = CreateWebRequest("http://ift.tt/1pLXQ3d", 
                                       "GET", 
                                       "somedomain.com", 
                                       timeout: (10 * 1000), 
                                       bindToIpAddress: "192.168.27.133" /*site IP*/);

        request.Accept = "application/json";

        using (var response = request.GetResponse())
        {
            using (var sr = new StreamReader(response.GetResponseStream()))
            {
                result = sr.ReadToEnd();
            }
        }

        return result;
    }
    catch (Exception ex)
    {
        return null;
    }
}