Several calls to Web service with Windows authentication from same method fails

I have a problem with a new web service that uses windows authentication. When a call the web service more than once from the client method, I'll get a timeout error. Surely it has something to do with the authentication, if I remove the windows authentication from the web site everything works fine.
WebService1 Shipping = new WebService1();
Shipping.Credentials = new System.Net.NetworkCredential(user,password);
DataSet AppCust = Shipping.GetNotApprovedCust();
// Next call fails

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Ted BouskillSenior Software DeveloperCommented:
Could you please provide the code for the WebService1 class?
Contento1Author Commented:
Ok, found it myself. if I use

Shipping.PreAuthenticate = true;

it works.
Thanks anyway.

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