Thanks for analyzing the file. I don't think the other mental ray examples you gave are related to this. Not to say that mental ray isn't the culprit but I'm not ready to give up yet.
Yesterday I identified some bad frames in my animation. Out of 2100 frames (4501-6600), 44 were bad. I set up backburner to re render these frames. The first one was 4624. This frame came out good, but the next 33 frames did not. Then the next ten rendered correctly. In all of the bad frames, the forest objects were clipped as if the camera was still positioned at the first frame (4624). To verify this, I moved the time slider to one of the bad frames (5209) and then changed to perspective view. Then I moved the slider back until the forest objects were clipped in a way that matched the bad frame. I happed to be frame 4624.
So, in this example forest pro interpreted the camera as being stationary even though it was moving. Then, after 33 frames, it corrected itself. What do you think? Is this forest pro or mental ray?