C# with ajax update panel and Cache object causing re-authentication

Question

Hi, I have a web application that uses an update panel to display a grid of patient data. That grid is updated every 5 seconds using an asp:Timer control. For authentication I use windows with impersonate="false" (in the web.config file). Also IIS 6 has been configured with no anonymous access and windows authentication. The grid data is retrieved by using ADO.net. The problem that I am having is that when I do NOT use the Cache object (the Cache object uses a delegate to repopulate itself every 5 seconds) to store the dataset then things work fine, but when I do use the Cache object then after a while the web application pops up a window that requires re-authentication (user and password). How can I prevent this from being happening, since the Cache object is so good for performance??