From my experience what I'm seeing happening is indicative of a memory leak. Not sure what is causing it though. After a random amount of time (5-20 minutes usually) it starts to lag, eventually ending in my movement completely locking up. I can't move or interact with anything but I can look around choppily. Running Ultimate pack on Windows xp. Happens on both single and multiplayer.
http://pastebin.com/embed_iframe.php?i=Z578XFNY
http://pastebin.com/embed_iframe.php?i=Z578XFNY