Miss Nancy: thanks for the tip. I usually do final renders with 8, 0.2, 100, and I'm fairly certain I still get these artifacts then... but Im going to check that in the scene for the above image.

Call me slow on the uptake (though my name is Izi), but it has just occurred to me that these lines only seem to appear on large background planes, such as on the sky dome here, or on backdrops.

pitklad: It's very kind of you to offer, but I have a feeling that Photoshop actions won't work in my Photoshop Elements.

Besides, it's so quick, it's probably not worth making an action for it: I just copy the image to a new layer, offset the background layer by 1 pixel vertically (up or down doesn't really matter), then merge the two layers.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)