Ok.. update.  I started a completely new document and imported each model individually into it, then rebuilt the scene from scratch.  I used a different terrain and the same realistic sky settings with a bit of haze this time for effect.  I have only one light source, a single sunlight, and the ambient set to 5%.  I started a new render and it is not only performing flawlessly, but it is flying by this time.  I am chalking this one up to some spontaneous goofiness on behalf the lovely C6 lighting system. :)