Why MS Office Word 2010 restart when 2 are open

Hi TR

I have a problem with MS Office Word 2010. Everytime I work on 2 or more workBooks trying to copy from another and pasting on another, my MS Office Word 2010, closes and restarts without saving, which is setting me back alot. I’ve tried, repairing/modifying and re-installing the whole Office package, but the problem persists. What else can I do to resolve such an issue. Please assist