It sounds like an earth loop problem through the Cable STB.
Is your cable STB connected to the cable system via a Galvanic Isolator ?
I will answer your next question before you ask it.. What is a Galvanic Isolator ?
A Galvanic Isolator allows the cable signal to the STB without the associated ground link to avoid ground loops or other electrical potential differences, the Galvanic Isolator MUST always be used when you are connected to a cable system.