My first thought is dampers, when (if ever) were they last replaced? Have any other parts such as the wiper blade, capping station, pump, flushing box and/or print head ever been replaced? If so, about how long ago?
The reason I think/ask this is because magenta and yellow pigment inks are always the thickest colors (due to the natural pigments used, and is true with all pigment inks), so restricted flow due to particle build up in the dampers will often show in these two positions first, indicating the dampers need to be replaced. I recommend you read our article on printer maintenance, which you can find here: http://www.inkjetmall.com/tech/content.php?133-Printer-Cleaning-and-Preventative-Maintenance, and if it's been 3 or more years since your printer has been serviced with new parts, I suggest replacing parts as outlined in our printer maintenance article.
I hope this helps. Please let me know if you have questions or there's anything further I can help you with.
Best regards~ Dana