Flickering final gather map is one of the main problem with animation in maya mental ray. This is due to the random sampling on the FG map in each frame.
Require some workround to elminate the flicker based on situation without using very slow high AA sample, points density, interpolation and rays.

1. Fixed objects moving camera
FG_shooter mel script – fix FG points
http://forums.cgsociety.org/showthread.php?f=87&t=965546&page=1&pp=15
tested, this works quite well so far for exterior scene

2. animated objects
Freeze FGmap every Nth frame & composite
http://asephei.blogspot.com/2011/03/maya-flicker-free-final-gather-in.html

thread on flickering render
http://forums.cgsociety.org/archive/index.php/t-866650.html