Epson GS600

Sounds like magenta is cross contaminating the yellow. Have you changed the maintenance parts recently?

 

Take a photo of the nozzle check and post it here. Also, take a look at the age of the ink in the GS6000.

 

I remember the canned profiles for these printers being wildly wrong. Pending the results of your nozzle checks, even perfectly working GS6000's have wild output on orange and green gamuts until dialed in correctly. I always recommend using the CONTONE driver instead of the halftone one, this usually gives the best results with minimal fiddling. Next, did you profile this in house or download a canned profile? I would completely disregard any canned profiles - I've never seen one accurate. You'll have to make your own. Let us know what develops and we can hopefully be of more help!