I have a CRT TV with only a digital adapter, but I've noticed this problem especially on The Science Channel and TBS the picture intermittently pixelates and/or freezes briefly. Also TBS keeps switching between full-screen and letterbox formats. I have not noticed this on other channels, but then I don't watch every channel. My service area is Albany, NY.
You likely have a coax issue...loose connections, damaged coax, too many splitters, etc. causing slight signal problems that manifest itself on one or two channels that you have noticed. Check that all connections, even behind the wall plates, are tight and that you don't see any obvious coax damage. Then make sure you don't have any unnecessary splitters or coax line running to "nowhere." If that doesn't correct the problems you will need a service call.