Can you help me with this cable TV question?

My cable was working fine up until a few weeks ago (we have been in our house almost 4 years). Then my Speed channel started getting pixely (not sure if that is the right word or spelling) and so did a couple of other stations around that number and one eventually went black. This is only happening on one tv, the rest work fine. So, first it was suggested to switch the box. I did, and it was exactly the same, no help.

Then a tech came to my house. He supposedly disconnected all of the cable in the central box/panel in my house but that TV was still on. I mentioned when I first moved in the cable company installed that cable outlet, because the one originally put in the room was at the wrong end of the room. He could not figure out how to disconnect the line, but said he could fix it if I joined comcast monthly insurance, or he could fix it on the side for not to much.

Aside from the obvious b*lls**t what do you think went wrong with my cable TV considering it had been fine? Oh, and he switched my cable box to another new one, because the one I got a week ago had no time on it, and the new one does not get Speed channel at all, he said it was the box, it all sounded like a crock to me.