QTR-Quadtone RIP

OK, I cleaned my heads, tried all the available dithering patterns, 
and still get mild vertical banding on H Photo Rag.  I've used an HPR 
profile, and also the standard EEM.  Both show the banding. (I also 
spent a lot of time reading up on the banding issues via search--maybe 
not enough, but a lot anyway).

Help would be appreciated.