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First thing to look at is Rapid Fail Protection on the Application Pool.
The default is to be enabled and to restart the pool after 5 failures

This is cute, but especially working with nooby developers it causes problems, because Response.Redirect or Response.End causes a thread abort error on the server if used incorrectly.

So what you might see is after 5 response.redirects the pool is restarted and all your sessions drop...

Another thing that might cause it, is your worker processes. Inproc sessions are not shared between processes.(Kinda logical, no ?) So if you need more than 1 you need to use a different session persistance mechanism.

Hope this helps :)