And to give you a short answer, yes.
Cable signals are good (in a perfect world), up until they come into your house.
It seems like you have really old coax wires inside your home. Although this is a silly idea, try and see if you can replace some of the lines inside your home with new ones.
Cable signals, especially hd signals, require a certain db level, otherwise you're going to start to experience tiling issues and image degradation.
Sorry to be the bearer of bad news but, at this point I would suggest bringing new lines into the house and do lots of wire fishing.
Unfortunately, the home I moved into is wired for cable via a maze of splits and extensions. The room in which I watch the most TV is at the end of the line after many splits.
In this room, I often don't get many of the features I should on digital cable -- the OnDemand works intermittently, and even some channels don't work. I had to cancel my cable-internet due to poor signal strength. Oddly, things work much better in cool weather compared to hot.
I'm considering a new HD TV, but wonder if its going to be worth the investment -- I'd hate to get the TV home, install the Comcast HD box, and then learn I cannot receive (adequate) HD signal.
Of course, neither comcast service nor a TV salesperson has any intelligent reply to this question.
Does anyone know if HD is signal strength dependent?
(Second question - does anyone have any theory why warm weather would reduce signal strength/increase problems. My theory is that the old copper cable in the wall expands during heat. I get much better service in the winter.)