ASP.NET AJAX and Forms Authentication generate a HTTP 401 sporatically

I have a situation where I'm using clientside ASP.NET AJAX requests to a WebService in the same web site. The WebService folder is protected by FormsAuthentication. It seems to work fine, execpt when I load test the website using VS2008 Test Edition, a majority of the web service calls are generating HTTP 401 responses.  Any ideas what I can look at?
Thanks! 
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