I found if I removed the last line of splitters (don't think they're necessary?) the problem was almost totally confined to the inner two belts, that come out of the Underground Belts. I see it about 2-4 times per second.
IMAGES
And the full image, including the gap-free input.FULL
I've also attached the save, you should see this happening regularly. Also rarely in the Iron balancer next to it but not nearly as often, probably because it's not drawing as much.