do you have FG on?

u also might want to increase the spread close to 1. i ususally use .9-.95

it might also be because its calculating the occlusion for every frame. so since you have max distance not at 0, at each frame the mesh is slightly farther or closer to the camera center of interest, so its calculating the occlusion differently on each frame.

yous hould, bake an occlusion map using transfer surface info [i think in maya. ] and use that map. it will increase render speed by 1000x

or leave max distance to 0