[Therion] Loch giving strange artifacts

Hello, in my loch model i get some strange artefacts. It seems that the further-away objects are rendered in front of the nearer-in-front objects. I would expect that the passages far away would be covered by those in the front of the camera (as is the case where i did not yet have scraps and only the centerline is present... look at the screenshot) What am i doing wrong?