PS:
A long shot: Preference File corrupt.
This file can go "funny" and is then responsible for all kinds of strange behavior of Lr. The remedy is replacing the Preference File.
BTW: Re-installing Lr ddoes not replace the Pref. File since it is designed to "survive" upgrades.
See here for where to find the Preference file for your OS: http://helpx.adobe.com/lightroom/kb/preference-file-locations-lightroo m-4.html
See here for how to go about Replacing the Preference File: http://lightroomers.com/replacing-the-lightroom-preference-file/745/
