>>To be clear : you mean that if you stop and restart the application pool then access the site from a new browser instance that the old DLL version is still being used?
>
>It is not that the old DLL will be used, it is that the browser falls in Waiting... time because IIS cannot respond to the new DLL.

Have you applied this: http://support.microsoft.com/kb/969612/
(See 961884)