Situation: Classic ASP application, using a custom Application Pool. Default settings.
On some IIS7 machines, IIS decides to serve only one page at a time.
If it isn't an issue with the debug settings, I've found that you can experience similar behavior when using session variables in ASP in IIS 8.
I don't know that this is IIS 8 specific, but "ASP guarantees that only one request from a Session will be executing at any time." http://msdn.microsoft.com/en-us/library/ms972335.aspx