problem with report viewer

I found a few topics about that problem but I'm not sure it solves my problem.

I have an aspx web page (ASP.NET fw2.0) with 4 report viewer.
Each report view is running a diffrent report.
in the code behinde I'm adding each report a parameter from my page.

Now, my site is configure to send me errors to my mail.

I've noticed that every time that I leave the page with the 4 report viewer open, after about 10-12 minutes (without toucing anything and without any refresh or postback) I get an error mail with this error:

Execution 'ucv5xtu2x2uvhfjmjkfmfy45' cannot be found (rsExecutionNotFound)

The page itself stay OK with the 4 viewer and the user don't know that there was a problem but I do.

Any thing know way is that?

I saw some thing about session of the report viewer but I'm not sure about that.

B.T.W
I'm still with sp1on my sql server, Does any body know if it realted to my problem?

Thanks!