We have a client who upgraded from 2.6->2.7 then straight to 3.1. At beginning any user would run into this freezing issue randomly. Then we copied the site to a different server and since then every new user would have his browser freeze upon exit the SCORM, or an existing user attempts a new SCORM package would cause that as well. We found that whenever the issue happened, the datamodel.php got pending in the background and that caused the browser freezing up.