In many cases it's in the source, and when it is, I find it can be reduced, but never really gotten rid of.
I have a GTX980 as well, but I can't see that as being the cause of it.
eg.
Disneynature's Monkey Kingdom.
At the end of the scene where the "troop" has finished raiding the kids the birthday party.
They're full of cake and junk food and are going to sleep after stuffing them selves.
If you keep watching, the night time sky falls and there is some (what I would call) serious banding.
If I set my resolution at 1080p in the NVIDIA control panel and choose chroma subsampling 4:4:4
at 12 bit, I still get it.
At those settings there shouldn't be any.
Doesn't matter what I choose in MadVR...
It gets even worse when I choose a resolution of 2160p as the chroma subsampling drops to 4:2:0
at 12 bit.
And again MadVR can do pretty nothing to correct this.
Now at 2160p, I was told that the best that can happen is 8bit (Sony 300ES Projector) even though I can choose 12 bit in the NVIDIA control panel.
So there must be some sort of EDIT anomaly going here _ so it's really hard to say
At any rate I am convinced that Disney used some pretty piss poor cameras here.
EDIT:
I just looked at your screen shot, that's not that bad, it's far worse in Monkey Kingdom.
As you said, it only happens in some movies _ so I'm going to say it's in the source and there isn't much that can be done about it.
What was the movie that your screen shot is from ?
I'll check and see _ chances are pretty good that I have it.