3ds Max :: Interpolate Over N Frames Creates Blotchy Results

I have an interior scene with an animated camera and a lot of animated lights.
 
I set to Project FG points from positions along path.

Divided camera path by Num. segments 49 (the animation is 1000 frames in total)
 
I then calculated the fg map creating one per frame.
 
When I come to render the scene if I leave Interpolate over n frame at 0 the scene is smooth but it will flicker as it will recalculate the fg per frame.
 
I know the point in in changing the interpolation over n frames to greater number than 0 is to reduce the flicker, however when I set it to 2 for example the render now is extremely splotchy.
 
This happens when I try rendering from 10 too (so that there are the fg file before the start to allow interpolation).