The reason you still have some interference from your cable box is not the coax, it's the box itself. The shielding in these boxes now a days leaves much to be desired. It is radiating from the circuit board inside (made in China) and with the lack of shielding in the case, it is still radiating some. You could try to place the cable box in a metal drawer and ground the input connector and the drawer to a good short ground.
