I think the flicker on the model's ear is the big clue. It's def the hair. There appears to be an outer transmapped sphere of hair around the scalp. The light's coming through and flickers because, of course, an animation isn't smooth movement, but a series of static pics. Try the same scene without any hair on the model and see what happens. How to reduce or eliminate the effect? Well, the things I'd try are increasing the Pixel Sample size to 5 or 6, tick 'Smooth Polygons' and reduce the bucket size to compensate (your cpu will love you for it!). 128 is pretty big, I'm thinking (won't affect the finished render, but will probably be slowing things down). Try 20 or 30 instead. Good luck.