I had a similar problem a few months ago, and never did find a for sure answer.
Trying to print grey was giving me a reddish brown color.
The printer was working fine as long as there wasn't any grey in the artwork.
I cleaned everything and the problem wouldn't go away.
Had a 440 black ink in it and when it ran out of black ink, put in a new fresh one, and the problem seems to have fixed itself.
(I think the 440 was a little old, Roland had given me two of them for some trouble I had, and it was quite a while before I used the second one)
Still not really sure if changing the ink helped or if it was from all the cleaning I had previously done.
So..if all else fails, you might try a new black to see if it helps.
I don't print enough for the 440's, so use 220's just to help keep the ink a little fresher.