the banding is on the screen. If it's a problem caused by the video card, I think that I should see the banding also in Photoshop. To see the PDF in Photoshop I simply opened the PDF page (with the TIFF image) that I exported from Scribus to try, without changing the resolution (400 dpi).
I tried, as you told me, to export the PDF page in another format (JPG). I imported it in Scribus and exported the PDF. In Acrobat Reader I have the same problem (perhaps worse). And, like before, there isn't the problem in Photoshop. I tried to save the TIFF in JPG, import it in Scribus and export the PDF. The banding is a little less but it's present in Acrobat. Also in this test, in Photoshop there isn't the problem.
It's true that a light banding there is in the original file (I'm a illustrator and always I clash with the problem of large raster gradients, I can only reduce the banding with a little of noise effect), but I don't think that Photoshop adds and views something that there isn't. I don't think even that it fixes the banding. It will be Acrobat that visualizes badly instead. A little odd!