Mrderrick and Fpeel have it right about the correct fix. The problem is that the cable TV coax sheath provides an alternate ground path for the AC neutral wire if the coax ground is not in close proximity to the common AC ground point. The Radio Shack ground isolator should help in any case. From my experience it won't completely eliminate it but will knock the hum down into the system noise floor. In any case insure that the cable TV coax is grounded before it enters your house otherwise you have connected a potential lightning rod to your system.
