To me, it looks like the part cooling is not as effective as it should be. In particular, I think that it's cooling one side better than the opposite side. I also have a Kossel XL and have replaced the part cooling fan ducting with something that provides air from more directions rather than just the single direction that the original duct features. I would say that improved it for me although I don't think I yet have an optimal solution. I often have to rotate parts on the build plate to get the most effective cooling. Unfortunately, that's not an option with your current model. In the past, I have just positioned a large fan off to the side of the build plate to move a large volume of air. Incidentally, what temperature are you printing at? Can it be reduced without the quality suffering?