Gutenprint problem on 3880

[peter moseley]

I've a fascinating and frustrating, hair tearing problem printing via Gutenprint through both Photoshop and Print-tool on my Epson 3880. Prints are of high quality except at very limited specific tonal values, for example at R59,G59, Blue59, where the result is so bad that the print has to be chucked. The attached image shows a stepwedge with only one step affected by this glitch which resembles headstrikes rather than the smooth color it's supposed to be. I've tried paper feed adjusts, head cleaning, head alighnment, custom paper settings, different dither algorithms and different media types, all to no avail. Any suggestions gratefully received.