We are getting drastic color changes from day to day on our Roland VG2-640. For example, when printing some posters with a large blue area, the color will be blue one day, and shift to purple the next day when printing more of the exact same RIP'd file. We did not change inks or media types between days, other than replacing the media when it ran out. The color doesn't shift throughout the day, only from one day to the next. Any advice?