Print quality is poor when printing flat colours

[craigeves]

Hi, I hope you can help. I wondered what settings you use to print to get a perfect print quality on flat colours?
I keep getting this (or banding) - never had the problem before upgrading my OS on my Mac and downloading the new driver.

Sorry, should say I have Sawgrass SG400, printing with the Sublijet HD inks on Subliflex paper.

Thank you

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