I usually encounter this problem when I try to open multiple links (in different tabs) on my site at the same time. For instance, I'll open the admin dashboard in one tab, while I pull up the plug-ins portion of admin in another tab. It happens randomly, so it's hard to test it out (e.g., by removing every single plugin). So I am hoping this problem looks familiar to someone.
I'm using Firefox 3.6.9. I've had this problem on previous versions of Firefox as well. I have not confirmed this issue on Firefox 4.0 b5 or other browsers. Indeed, it seems that while the pages are hanging, if I open up more site portions in new tabs in Firefox those will also hang. But if I open, for example, Chrome during that same period of time, I can access the site just fine.