You can try (in order of increasing price and effectiveness)

RadioShack RF Interference Filter 15-579
Xantech 63400 Ground Breaker
Jensen Transformers Cable TV RF Isolator VRD-1FF

But I would have the cable company come out and make sure you cable feed is hooked up properly. The experts will hopefully correct me if I am wrong, but I believe that they are supposed to tie the cable ground to the same ground as the electrical system. Or something like that.