Hi Michael,
It may be that you are encountering QuadToneRIP limitation to printing near to the bottom of the print. The margin area has always been about 0.56" or about 1.5cm but it it possible with new OS X this margin may need to be enlarged. If this is just the lower edge you can tape a leader to the bottom to extend the size of your page layout so that QTR does not begin confusing the end of the image with white pixels in its dithering algorithm.
Jon