I tried many things, GI, more atmo samples, AA, animation settings, subdivide etc.
With some it looks less sporadic. But still it is like the sampling is all over the place.
If nothing comes to my mind this is so far i go. The object polygons could be a problem (doubt it but just in case) for example.
I will triple them and try ones again.
Edit: No. Same results.
If you want you can try the same scene here too: