I've had this exact thing happen when the saved image's alpha channel gets messed up, since that's what Bryce uses to remember where it left off with the render. Same thing when restarting a scene render with sun soft shadows or light gels (they're not saved with the file). Don't suppose that that's a render restart? One thing that may help is to solo just the buildings or the cubes or the mountians and do a render and see if it still happens. Sometimes this can narrow things done to something specfic that's causing a conflict. Since soloing dosen't effect lights, if you've got any additional lights in the scene trying contol-X ing them - if the lights are Ok Control-C them back into the scene. Don't know that this will help; Dan