You may have the yellow sucking up some red.
Bad caps, and/or big difference in red and yellow cartridge ink levels are a couple of reasons for this.
Do a medium clean, or print some big squares of yellow. 4' x 4'... You have to use up the ink in the head, damper and line. Hopefully it has not reached your cartridge.
The sponges can swell up on the caps, causing them to touch the head and bleed ink out which can be drawn into the yellow channel.