System.Net.WebException: The remote server returned an error: (407) Proxy Authentication Required.

Hello, I am using XMLTextReader to read in an xml file from an external website. Randomly I recieve the Subject error. After doing some research and very little understanding on the subject, I added the following chunk of code in attempt to add a webproxy but I still randomly receive the error. Here is the code:

System.Net.
WebProxy proxyObject = new System.Net.WebProxy("http://proxy.ilmgroup.com:8080/", true);
System.Net.WebRequest req = System.Net.WebRequest.Create(http://www.treas ...