QTR-Quadtone RIP

I have one printer set up with piezography inks that I use for B&W. 
For colour I am using an R2400 with Image Specialists pigment inks. 
For a bit of fun (or so I thought) I would try and create some curves
for the Ilford Gloss on the R2400.  

I followed the instructions as per usual, using the
UCpk-IlfordSmooth-neut curves as the starting point.  I found that I
needed to use 2880 to avoid some macro banding.  The first set of
linearisation parameters that I derived were not always increasing (or
was that decreasing), and QTR refused to linearise.  

I decided that I had set the ink limit too high (60%), and repeated
the ink separation calculations at 55%.  I honestly thought that this
gave me the right sort of black without overdoing it, and I even
measured this with the i1.  However the curve I produced was rather
lumpy, as per this diagram:

http://www.ferdinand-paris.com/2400_bumpy_curve.gif

I guess I should drop the black limit down to 50%, or are there other
curve parameters that I should be playing with, like the LK & LLK ink
limits or the "Grey Curve" / "Toner Curve x" Tabs?

What does this lumpy curve tell me about what I might have done wrong?
 I've successfully created curves on matte papers before and not had
this problem.

F_P