Further to my previous post:
Your problem could be due to a corrupt Preference File. This file can go "funny" and is then responsible for all kinds of strange behavior of Lr. The remedy is replacing the Preference File.
You said that you re-installed Lr. But re-installing Lr does not replace the Preference File since it is designed to "survive" uppgrades.
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/