If you have a set top cable box - then call your cable company. I had worlds of problems with my cable company when I tried to explain to them that the image was FINE two days ago, but now looks like crap. They redid all the cables that I had done and it looked the same... Duh, cables usually don't just go bad.
Turns out I was getting to MUCH cable signal at the house and they had to notch it back some. They got it fixed, but obviously wanted to blame the issue on me first. If you have tried everything you can think of, call your cable provider. It is their responsibility to get you a normal looking image.