This may help and it might not but, worth a shot. I have heard that many cable companies use a PTFE shielding in the cable itself. This can be also found as teflon tape or plumbers thread tape. It is white PTFE tape. I used this to shield a cable I was having hum problems with and it helped it significantly. Give it a shot. Teflon tape can be found at any hardware store for cheap.
Andy
Andy