Presuming that your downstairs digibox is working ok,try swapping the boxes round,(unplug the mains leads, telephone leads and the coax f-connectors) take the downstairs box upstairs,and the upstairs box downstairs, set them both back up again.
If the fault has moved downstairs then its that particular digibox that is faulty.
If the fault remains upstairs then it could be the coax wiring to the upstairs digibox.
Sky installers usually fit a twin LNB on the Sky Dish so everything on the Dish is shared by the 2 digiboxes,so with the downstairs digibox working ok everything on the Dish sounds ok.
Hope this helps.