Gradient background flickering?

[Alliat]

So I've made my first attempt at animation (not counting back in '94 when I made some 3D objects spin lol) and I'm fairly happy with the outcome but one thing is puzzling me and that's the sky background which has this light blue to blue gradient. It keeps flickering in the beginning of the animation yet there are no keyframes present at the frames where the flickering occurs.

You can see it here: http://this.is/alliat/files/turmalin.mov
(this is nearly 30 mb so loading may take a while)

-There are a few tweaks I need to do with this one as well. The mountain has this "hemorrhoid" on the right where the glacier ends. The steam isn't supposed to follow the lava all the way down, only as far as the glacier goes. And the steam entry is a bit off. But those all I can fix already, the flickering is what's puzzling me.

[Alliat]

Oh, and p.s: this is just a part of the animation, i just decided to cut there because of the 3000 frame limit. Next scene will keep scrolling down to reveal a cavern and I'll go from there.