randomly changing user

Hi,
I have an application in ASP.NET 2 configured to impersonate, cookies
enabled and Windows NT authentication only for whole application (anonymous
for one or two files inside).

On every page I write currently logged user and I've noticed recently, that
this randomly changes during session. Well probably it is not randomly but
it changes heavily from user to user, maybe last accessed one or something
like that. Please help what is causing this and how to avoid.

The another problem which I believe is caused by the same mistake is, that
if I open the one file with anonymous access (from another session), I loss
my current identity in the current seesion. How to retain it?

Thanks,
Jan