My cable comes in at the electrical service entrance. It used to go through a splitter for branches to up and down stairs. When I had this problem, I disconnected it at my DVD player to no avail (don't you remember that post of mine \:\) ?). When I pulled it out of the splitter, the problem disappeared. Then the cable company installed the isolator and they also took out the splitter and put in an amplifier. Since then, I haven't had a problem.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated