Hi Dan,
Hopefully that will fix it.
My 9900 does this also, and also with no rhyme or reason, a channel just will stop printing, sometimes during a job, other times right after. In fact my 9900 always has done this to some degree, (early on with orange and green), now with the more important colors.
I had the head replaced right before the warranty expired, along with the capping station and dampers, which seems to fix the problem briefly, but it's back now.
Still get a good NZ ck, so like you I have too much ink on hand to replace it with the 9000 and will keep on going with it.
Last time I checked the warranty from D1 was either 90 or 120 days, I can't remember which.
Paul C