"Multiple page" mode
I've had this problem off and on for a while. I have the system set to show all the posts in a thread at once, and I can scroll around on them. But for some reason, sometimes the system trips into "multiple page" mode, where I get two posts and then at the bottom page numbers (1, 2, 3,) to go to the NEXT two posts.
I can usually fix it by restarting IE, but it's a pain.