Before calling Comcast I'd like to get feedback.
-cable
-one box per tv
-two tvs
-one works fine
-second one audio and visual is breaking up, piexels? across the screen are distorting into larger sections along with the audio. Not the whole screen, just sections
-at first the problem was fron non-local channels, now local channels are starting to do the same
-is it the tv or connection?
that sounds like something they call "tilling" … I've been fighting that for nearly a yr now … the only difference is it's very intermittent and it affects both TV's
the typical fixes, are replacing old in-home wiring, the boxes themselves with new/updated cable boxes, the outside connections, the pole to house drop.
all of which we've done … it got better, then returned to the intermittent problem …
one of the techs that has taken an interest, told me that there is some equipment on the pole that does something "magic" ( the " " are mine) to sort the signals as they come in .. he said he saw a lot of break up in the "packets" (whatever that is) at the pole … he didn't have the correct piece in his truck to fix it and would pass it along to the correct tech for repair … as intermittent as the problem is, the next tech couldn't isolate the problem, so couldn't "fix" it …. so on we go 
good luck
Hmmm, ok, I'll go check the connections when it gets light outside. If that doesn't work I'll call and mention tilling. The boxes are pretty old, too, and changing them would give us better viewing options.
Bad connections cause resistance in the lines and that can cause poor signal. Try disconnecting the line from the back of the cable box for about 30 minutes, reconnect it and see if there is a change.
If there is, replace the cables with new RG6 labeled coax cable and new junctions and see if that fixes the issue. Make sure all of the connections are snugged up (inside the wall plates as well). If not, complain to Comcast and have them replace the connection at the pole and the line to the house and/or the problematic cable box.
That could be caused by a bad connection on any kind of digital TV system.
I've seen cheap coax splitters cause issues like that before.
I'd suggest a new cable. Go get some coax, a sharp knife, some cable ends, something to crimp them, and don't cut yourself.
Being the resident Comcast Tech, I'll throw my $.02 in.
There are many things that can cause tiling. Loose fittings and cheap splitters are the biggest culprits. Next up are bad fittings that are of the "Spin on" or crimp style (compression style is the best). From there it could be bad wiring inside or outside the house, including the drop from the house to the pole or pedestal. Last but not least, it can be a bad cable box or a main line issue.
Seeing that one box is working, but the other one is tiling, usually means there is something wrong with that particular line.
My suggestion is to make sure all connections are tight. Standard fittings are 7/16" in size and just take a wrench and make them snug, not torqued down to the point of snapping off. 
If everything is tight, no wiring looks obviously damaged, and you're still having problems, call to have a tech come out, it may be something out of your realm.
Ditch cable and get Netflix?
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