I've searched around this forum and several others to try and get to the bottom of this but nothing quite hits the nail on the head.
I am currently panning a camera through a scene of mine using a volumetric light and a fog based sky for atmosphere.
After I think about 20 test renders now I still cant get over, what must be a very simple issue to do with flickering in the scene.
Part of this problem was resolved with sample distance but, not all of it.
The images provided are frames 218 and 219 to show how the atmosphere changes dramatically.