Digital BW, The Print

I've just purchased a R220 with the idea of ordering MISPRO inks and 
Ebony to do both color and B&W in the same printer with matte paper 
and using the BO mode for B&W.

Before changing from OEM to MIS inks, I'm testing BO printing first, 
and I'm really impressed with the look of BO in most of my prints.

However, I've found an anoying horizontal banding which is ONLY 
visible in homogeneous zones (e.g. skies) which are mid/dark, not in 
highlights or very dark zones.

I've observed the same problem in two R220s, with three different 
papers, and ONLY in BO mode. Color or grayscale (using all inks)
homogeneous zones show no banding at all.

I've tried different things to solve BO banding, but none of them 
gave a significant improvement:
0-Head cleaning and nozzle check (no problems observed in the latter)
1-Head alignment
2-Low alignment numbers (#2)
3-QTR with all the dither parameters
4-Even resolutions (e.g., 320 dpi)
5-Unidirectional and Bidirectional printing

The bands are not void gaps, just periodic lines less dark than the 
background, separated by about 3 mm.

This problem makes me to reconsider my initial idea of doing B&W and 
color in the same printer, and go for the UT-R2 solution in a 
dedicated B&W printer, and who knows for color...

Any help will be appreciated.

Thanks in advance,

Fernando