QTR-Quadtone RIP

Any recommendations on how to fix small bumps that persist after linearization? By `small bump' I mean a 2% or so deviation from the ideal linearization for any given point on the line. For example, I did a warm Eboni-4-Plus  curve for Alise Natural on my Epson 1400, and the curve is perfectly linear (i.e. the points deviate less than 1%) except at the 50% and 80% patches, which are 1.5% and 2.5% lighter than ideal, respectively.

I grant that these two small bumps might not be noticeable in prints, but I'd like to get a perfect curve if I can.

Thanks, 

Mark