Tiling problems for me too. I have a TV with one CC, and (2) S3 each with two CC. The TV with the one CC seems to be the worst. Nick, Speed, HGTV. My HD channels seem to be OK. The S3 units seem to "deal" with the problem better. Currently, the S3 units are not tiling on any channels, but they have not been totally immune. For awhile, Speed was unwatchable, and ESPN2HD had problems.
My Mother, who lives about 5 miles away, has the same problems.
I have gone through so many CC swaps, you would not believe. I've had line techs out here and my signal strength is fine.
The most amazing thing is that my Mom has had her setup for several months, and she had absolutely no problems until maybe a month ago. I had my setup for a few months without any problems, until again, maybe a month ago. So, something definitely has changed on the Comcast side of things.
I am so fed up with this unresolved issue that I am giving myself on more month with them to see if it clears up for a respectable amount of time. It will be really bad for awhile, then for a few days, no problems. Anyway, one more month, then I cancel Comcast service. I'll get by with OTA and Amazon unbox.