C# - Thread.CurrentPrincipal Claims Incorrectly To Be Anonymous?

I'm seeing requests on my server that appear to be made by an anynomous client, although I'm certain they were made by an authenticated user - I have fiddler logs showing that the client sent valid asp.net auth cookies, and server logs indicating that the cookie arrived and is valid. The problem has been observed across all browsers.

The flow of data is:

User visits login url, on a machine that's part of a webfarm (all with the same machine, decrypt keys)On successful forms authentication, user is redirect to a restricted url, their homepage Homepage renders correctly, knowns the identify of the user, and includes javascript to perform 7 asynchronous post-backs to get additional data Javascript kicks off 7 http.get requests, hitting different machines in the webfarm (assume round robin)Server validates request: ~0.01% fail to authenticate.

Now into details:

A very small number of the asynchronous requests land at my server (with evidence they were not manipulated or faked) and appear to be anonymous. Of the 7 requests made, some number may or may not work (ie, 5/7 will succeed, 2 will fail). There doesn't appear to be any patterns in success / failures. In the cases where my requests appear to be anonymous, the CurrentPrincipal.Identity records:

Thread.CurrentPrincipal.Identity.IsAuthenticated; // false
Thread.CurrentPrincipal.Identity.Name; // null (or empty, unsure off hand)

Dumping the http.context.request.params collection to a log file, I'm able to see the following relevent (and scrubbed) properties (complete params below):

[code]...