Digital BW, The Print

Roy, et al,
I love QTR, and mostly the RIP has been flawless. However,  I have noticed in photos 
with large, very dense black areas, vertical lines (perpendicular to the print head 
direction). The latest photo had only 2, maybe 1" apart, but I have seen more and 
closer spacing. I saw it on Kirkland paper, but it persists on Epson Premium Glossy 
as well. If I print the photo in color (almost black and white in the latest case, as it 
was of a snow sculpture at night) using the Epson driver, I have no lines. 
I'm setting QTR to "1440 super" and "better" rather than "faster."
Is this ever a problem for others? Is there a setting I should change?
Sarah