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.
IIS7 is most certanly multi threaded, so I would guess there is a problem with your app.
You mentioned ActiveX to load a page from same server - maybe this ActiveX isn't free threaded and this causes every page that uses it to run single instance?
BTW: Web Garden - same server useing multiple processes - cannot use inprocess session Web Farm - multiple web servers