I have lived in several homes/townhomes where the ground difference was 6 mv or more and it is always the first thing I check. In my current house I had an issue and using a coax isolator did not solve the problem. I ended up using a 3 pin to 2 pin AC adapter on the AC for the TV. After unplugging and re-pluging each attached device, it was the Hitachi Plasma that made the difference. Since both cable and internet are coming on the same cable and I had a HTPC attached to the TV directly, I suspect I was getting a ground loop via the PC and its network attachment back through to the cable system.