Odd Portional Screen Flickering

I'm having an issue where a portion of my screen flickers between the correct image and a darkened image. The scene below shows the effect I'm seeing. This scene is made up of a few different cameras (and I've tried turning a bunch of them off just to see if they were the ones causing it, but none of them made any difference). Some of the camera's rendering areas (i.e. clipping planes) overlap, but all have different depth values.

What I've noticed thus far is that this happens most often when the y-axis of the camera and the y-axis of the Earth (North Pole) are roughly lined up. If I rotate the camera a few degrees, the effect disappears. The closer they are to alignment, the greater the portion of the screen (but it still flickers, this isn't a constant effect).

Another thing I've noticed is that the closer the Earth is to the edge of the screen, the more likely I am to witness the flickering, and that only the renderings of certain cameras are affected.

The Earth uses a custom lighting model and shader, but I believe I was seeing this before when I was using a simple Diffuse / Specular shader.

Does anyone have any idea what's going on here? This has plagued me for a while now and I haven't been able to resolve it!

I managed to capture one frame when it flickered below (the dividing line isn't always through the center of the Earth, I just happened to capture a frame where it roughly is):