unable to render

i am trying to export a 15 second video.  All of the sudden i am getting "unable to render" errors every other time i use Rush.  Used to work perfectly and now it only works 50% of the time. No idea why.  The video is just a basic MP4.  I can play it in rush, i can play it on my iphone I can play it in windows media player, but i cannot export.  Why is this happening and how do i fix it?