Please see the screenshots and explanation a forum poster has put on a Gigapixel thread I started over at Luminous Landscape. He is experiencing horizontal lines and other artifacts in his renders:
I reviewed the example posted. The only way we currently have to correct (i.e. with little change) would cause the processing time to slow even more. As I have stated in other posts we are working on an update to address such artifacts, lines, and banding that some images encounter.