So far I'm only seeing this annoying problem in 2014, not 2013. Difficult to say exactly what first causes it, but typically I'll be previewing in full screen mode (Alt+Enter). I use the Arrow Up and Arrow Down keys to start and pause play, all fine so far. Then I'll use Esc to return to the timeline and make an edit of some sort, usually a D or U. Or a T and then Ctrl+Del. Then I'll return to full screen previewing and (sometimes) the arrow keys will now no longer work. Worse, even back in normal preview mode they won't work after that failure. The only way I've found to recover is to restart MEP 2014.
--
Terry, East Grinstead, UK