Another stab in the dark:
If you have a pop-up blocker installed it may be causing this if the method it uses to block pop-ups replaces the URL called by the pop-up with the URL of the current page. So when you hit the BACK button you are really going back to the pop-up which is redirecting you to your current page, so it looks like nothing is happening.
I've tried everything except a jack-hammer on this thing. I think I'm gonna have to learn to live with whatever is wrong with it. I sure miss the way it was though.