Maybe someone here can help me get to the bottom of this.
I've just designed a brochure in Illustrator, the problem was a light green background created in AI with a raster image set on top. The raster image is set in a vignette that fades from the image to the exact same green background colour, this was done in PS. No transparency in AI involved.
When we were proofing at the printers, they wanted the AI file converted to EPS, then we test ran a print, which resulted in a set of soft dark lines where the green background of the vignette was over green AI file. The image was fine, although the lines could have been present I suppose but masked by the image complexity. I tried the file embedded as a .psd and as a TIFF and the problem was the same. The printer didn't know what was causing it so suggested we import the EPS into PS, then brought that PS file into Freehand where they do their layouts.
This step resolved the problem.
I'd like to understand what was going on for future reference. The banding error was not appearing on my consumer printer.