If I'm not mistaken, cameras and lights with fog don't always work so well when they intersect. Check to see if your camera is entering the cone of the light during those 15 frames.

If you MUST have the camera go through the light fog, I sugguest making a particle system of fog, and shine a spot light on it instead of light fog.

Kind of a pain, but it should work. Then again, my theory about your problem might not be right.

Let us know.

-dann