Did you find an answer to the problem somewhere yet? I read up a bit on physical fog, and I can't really think of what in the node itself would cause the banding pattern. You try a volumetric mental ray material for fog instead, but that's not really the correct fix to the problem

Another thing that struck me is that you might be able to get some help on a site called CG Architect if you posted it there, your scene is an exterior and they have a lot of arch viz professionals that give you some good anwers. Most people on there use max and vray, but there's a lot of mental ray and a bit of maya so someone there might know what it is straight away.

Nilla