It could be the cable but there is no such thing as 'Digital Cable' (Nor is there any such thing as a digital aerial for that matter.)
It is probably the length of the cable run but it could also be the connectors.
The best fix it to install an aerial booster as close to the aerial as possible and then run the cables from that.
You MAY get away with an aerial booster next to the upstairs TV. Plug the aerial cable into the booster and then the booster to the Set Top Box.
T
February 2010