Digital BW, The Print

This is wierd,

I'm making small 4.75x3.25 BO prints tonight on PR, BO with Ebony.   I've only 
made prints this small before on my 820 printer and in color.

Mostly in out of focus areas I see  I'm getting fine lines running vertically in the 
direction the print head moves.  Small fine B&W lines.  Maybe they are made 
up of very fine lines of dots and not solid lines.  Either way to the eye they look 
like lines.

In the image size window I changes the resolution from 360 to 240, this 
worked on small prints with the 820 if I remember right.  Didn't work here.

I printed at 2880 and than went to 1440.  The print got contrastier but the lines 
remained.  I've never noticed this in 5x7 or larger prints.

What is it?  Ideas?  Its like the printer doesn't want to make pritns this small.   I 
could add I saw these on prints with no USM used at all.  

Thanks 

Neil