Quote - two rhings you could try.

1. Set your render setting to the one just before raytracing. Render without raytracing and see what happens.

2. Switch to wireframe display and see if those defects are lining up with an edge in the mesh.

3. will show if its a raytrace issue.

Ah, now we are getting somewhere! I set it to the level below Raytracing as you suggested and the render has no artifacts. And the "stitches" are exactly where the wireframe display's grid is, although the entire wireframe is not stitched, as you can see in the photo. I don't understand the implications of this, however; they seem kind of dire.

Thank you for helping.