It could be a couple things. If your color setup is CMYK x 2 and you don't have the light cyan, light magenta, or light black, grays will always look a little more grainy than if you did. Make sure you run the drop position alignments and get them as perfect as you can. Do the same with the print feed compensation alignment. Try different profiles and see if they do any better. Turn the heat up as high as it will go without buckling the media. Turn the motor speed down to normal if it's on high. Make sure your room environment meets the specs of your printer as well.