Whenever I had the very same issue that drove me nuts for several days the problem ended up being the whole house grounding rod, specifically the ground wire from the rod to the electrical meter box ... just for grins and giggles I replaced the rod and installed new wire and connectors and made sure the outside cable coax splitter and box was properly grounded, haven't had any issues in the past four years since I did this.
IMO to nip a grounding issue in the bud you need to backtrack from the primary grounding source back to your components and not visa versa.
