Digital BW, The Print

I'm using an Epson 1270 with MIS VM hextone inks with an MIS CIS. 

Using Paul Roark's warm curves (Mac), prints look great. When I use 
the cool curves there are always slight banding lines running 
throughout the print.

Nozzles checks are perfect.

What could be causing the banding with cool curves?

Thanks for any info.

-Elmer Strueh