Cookie exiration rules, that seems a likely explination. I am not a techie, but does having lots of active member sessions going on at the same time take that much extra system rescourses? A hour before a session ends would make a huge difference to me.
If it's sometime I really want to take my time with, I too will open a second unique AudioHolics session and leave it open to the post in question and I go back to it later. I have not tried opening a third session. It's a pain in any case. Maybe it has something to do with the AH system resources.
Nick