QTR-Quadtone RIP

I'm printing a number of images on a 2200 with largeish areas of solid 
black and, to varying degrees, those areas are showing very narrow 
horizontal bands which appear like small scratches. At first I thought 
it was something wrong with the paper but I've now tried three different 
papers all with the same results.

I've posted a scan of one of the worst affected (it's about a 1 inch 
wide strip in the original):
http://www.glassman.mistral.co.uk/qtr/qtr_banding.jpg

I'm using 1440 super and have tried Eventone & Ordered dithers - the 
results are slightly different but just as unuseable. I've also tried 
different ink limits and various profiles but all with the same results. 
I'm using Eboni Black & UT-FS & FSN inks.

Looking back through the archives there was reference to a similar 
problem here:

http://groups.yahoo.com/group/QuadtoneRIP/message/937
http://groups.yahoo.com/group/QuadtoneRIP/message/1099

Any advice much appreciated.

Howard