Digital BW, The Print

I'm printing with vm inks on an 870 from a windows machine, and I'm 
using Paul's 870 curves.  For the most part, my prints have been 
excellent, but two prints have shown fairly serious posterizing 
toward the dark end of the spectrum.  Interestingly, this problem 
shows up with every single partitioned curve--that is, every curve 
except the simple, grayscale curve (gs1).  I've tweaked the curves so 
that I get a very smooth, even, 21-step wedge, but the problem 
persists.  After printing some very steep gradients, it appears that 
the posterizing occurs between 95 and 96%.  It seems that the black 
ink is being fed in too suddenly.  I can smooth things out by making 
a rather drastic cusp in the blue curve (lightest ink) at around 98%, 
but it's really hard to control photoshop curves on such a small 
scale.  Furthermore, such an extreme curve produces artifacts, 
especially when I try to adjust the other inks.  I should mention 
that these steep gradients print perfectly from a grayscale file with 
the gs1 curve, but if I convert the image to AdobeRGB (or sRGB) and 
use the gs1 curve (or even with no curves at all), the same kind of 
posterizing occurs as with the partitioned curves.     So maybe the 
problem has to do with printing from an RGB file.  Does anyone have 
any suggestions?

McKenzie