Report Sometimes Just Doesn't Show

Hello,

Since a week we have a strange problem.
We are using SQL Server 2005 with SP2 on a Windows Server 2003 updated last time in November. We have 2 servers like this, one is a virtual machine and the second one, a physical one.

Since a week, sometimes, the reports in the web report viewer just don't show. They keep loading for hours without showing anything. Beside, everything's fine, the server has plenty of free resources (CPU, RAM...), Visual Studio works normally, SSMS too. We can even go in http://localhost/reports and http://localhost/reportserver and see the folders, files etc... but when trying to load a report (what ever it is, even a small one) nothing comes, not even the message "Report is loading".

It happens not every day, sometimes two or three times a day at different hours.
In the Windows event viewer, nothing.
In the SSRS logs, nothing (as far as I can understand these logs).

It looks as if a job or a process was running and was blocking the reports or a service which would stop automatically.
To have it work again, a reboot of the server works or within 2 or 3 hours, it comes back to normal state.

If someone has an idea of what can be the problem or some tips, please feel free because we are running out of ideas.

Thanks in advance