I am having similar issues, but can't seem to resolve it by changing any render settings.   The images below show a viewpoint above a viaduct that diverges into two branches. Each of the viaducts is generated using a nurbs curve (in an extrude along path).   In wireframe everything appears as it should - nice smooth curves, and the point where the two viaduct parapets converge (near the bottom right of the image) is correct - they join in line with each other.   How