Digital BW, The Print

Hi, I have recently got the UT2 ink for use on my old 1270.. 

I am using the 1280 driver and curves to print, and everything is fine
for about 15-20 prints..

Then all of a sudden the prints I made from yesterday started of have
very fine horizontal lines running across the whole printed area.
(It's almost like clogged nozzles.) They are very visible in
medium-grey area.

But, the nozzle check is 100% perfect. A few cleaning cycles seem to
have reduced the problem somewhat but not all..

And printing the purge-6 pattern shows that the right 3 inks (the
colour ones) shows that pattern somewhat.. the left 3 seems to be
perfect..

Can someone help?
Thanks
Michael