Digital BW, The Print

I am using a linearized version of Paul Roark's QTR profile C-3MK-
IlfordGoldFB-GL-1PK on A4 Ilfordgold Fibre Silk and MIS inks/glop. I 
am using MP-PK in the PK position. All other inks as described in 
Paul's published PDF workflow.

When printing I specify 2880 and Uni-directonal.

I am achieving a D-Max of 3.15 - fantastic result. Thanks Paul.
(this was measured with my Colorvision/Printfix Pro spectro on a 
black and white test image available at www.northlight-
images.co.uk/digital_black_white.html).

My only problem in an otherwise perfect workflow is that I am 
experiencing very subtle vertical banding (about 3mm in width 
(alternating dark and light bands) on A4 paper) in the same direction 
as the longest side of the paper. The banding is only visible in 
constant mid-grey sky tones and while subtle is enough to put me off.

I have tried adding grain to the sky (using Grain Surgery) but this 
has not made a difference.
I have checked my nozzle print pattern - no problems there.
Any other thoughts on what I can try?

Thanks
Jim