Cable TV troubles, anyone?

Last fall we "upgraded" to Comcast's digital cable and then, this winter, added HD.

But ever since -- and especially since adding HD -- several channels have been unwatchable because of something called "tiling." The picture basically is fragmented into little blocks that dance around on the screen. It only happens on some channels, and it comes and goes on some of them.

A few weeks ago we had a couple techs out to the house and they told my wife there was some sort of general problem in the area that they were working on. They also said we had a weak signal and might need to have our original (15-yr-old) coaxial cable into the house replaced, or maybe some of the internal cable.

Since then the problem has gotten WORSE, with ESPN among others barely watchable and the vaunted Comcast "on demand" service unusable because of this "tiling."

My question to you all is: Anyone else had this issue after a Comcast upgrade? I'm wondering if it really is a problem in our area or just at certain houses. If you have had this problem, how was it resolved?