Is your CIS dye or pigment? Dye is preferred, but people here have used both. Ghostscript is a free RIP software, but I don't know much about how to use it yet or whether you can get it for Mac. However, you may want to do a few tests with different paper settings. Try glossy, luster, and matte paper modes. It could also be your films are not compatible with the ink you're using.