Search Engines - Error with rewritten Url

Hi,

I have implemented url rewriting for my site.
Its working well most of the time. but some times, It returns the 404 error.
then I refresh the page and it loads the correct page.

Environment: VS 2005,IIS 6,VB.NET,SQL Server

Thank you
Nagaraj

It is may be interesting with the Session Time Out process. I think, if the time cuts off the current process than main stream could not reach to the protected domain sources.