As soon as I saw the image I said Sun Soft Shadows! I've had this happen many times. Turns out that the setting for the sun SS doesn't get saved with the file. So it has to be turned back on everytime you reload and you can't leave Bryce or the scene until it's through rendering. Nor can you batch process it. You can still pause it though. A quick way to check is to make sure it's off and do a plop render - I bet it'll go away. If not, then I don't have a clue. btw - I've stopped using Sun SS altogether partially because of the above problem and also because the SS effect level is fixed at 100% soft. HTH; D