Digital BW, The Print

I've recently started experimenting with the Harrington Quadtone RIP v2. I'm using the 
latest beta on an Epson 2200 with the standard Epson inks. Generally the results are 
outstanding but I continue to have one show-stopper problem: vertical banding in 
dark areas of the print. This seems to be specific to the QTR RIP - the same image 
printed using the epson driver shows no such banding. 

It seems to be independent of resolution. I typically use 720x720 but see it in higher 
resolutions also.

I originally thought that the banding was due to excessive ink in dark areas. I tried 
lowering the ink limit but the banding remains.

I've posted an example here:

http://home.comcast.net/~baur/banding.html

Is this something others have seen? Any help in taming this would be appreciated. (I'm 
trying to get a couple of things printed for a show and this is driving me batty.)