I have the same issue. I have noticed while watching the image render, those black lines appear at the edges of the little render box ( I'm not sure if thats what you call it but the image renders one little box at a time) maybe trying a smaller bucket size (under manual render settings max bucket size) I don't know if that will fix the problem, I usually use the healing brush on them in photoshop. 
I will be curious to see what others say, thanks for the post.
Chris