Come to think of it, I used to have streaks of slightly lighter color appear in my renders... it was always at the top of the bounding box that traveled across the render window, and it kind happened at random. I always postworked over them, and didn't pay 'em much mind. I haven't rendered inside Poser in a long time, so I don't know if it still does it, or this was an old problem.

There does appear to be something going on related to that render box thingy.