QTR-Quadtone RIP

I'm trying to use version 2.6 and a 3800 to make digital negatives. I
can't seem to get even highlight densities (skies mainly)to print
without odd very random stripes of heavy magenta. I'm using an even
dark ink mix of 15 (black, cyan and magenta) and 8 yellow and 32 K
boost, but every so often I'll get these 1/2-1" bands with way too
much magenta in the mix. Sometimes these stripes will alternate the
length of the sky area, other times there will be a single very wide
stripe at the end of the same sky area. The ink level on the printer
confirms this- the magenta cart is half as full as the other dark
inks- which suggests it's happening more than I'm aware of.. I've
tried changing the dark ink limits slightly to no effect. Any ideas
much appreciated.