That's a good question. They are vertical bands that appear in prints on Pictorico. They don't show up at all when printing on paper (with qtr or through PS).
When I print through Photoshop, they show up in even-toned (usually lighter) parts of prints (e.g., grayish clouds). They are between 1/16 and 1/8 of an inch thick. When in PS, I have been able to make them disappear when I increase the media thickness setting, set printing to unidirectional (turn high-speed printing off), introduce a delay in printing each pass, and by reducing the paper-feed adjustment.
I am planning on creating a custom paper type that incorporates these settings to see how they work with atr printing a digital negative. I cleared the heads and did a head alignment about two/three weeks ago.
Thanks,
Brian