I doubt it's the TV. You're using a hi def cable TV set top box, correct?
The next time this happens, if you can do it quickly enough, press the MENU button on the cable remote to bring up the cable box guide. Usually, on most systems, the guide comes up with the TV picture you were watching in a small window on upper right corner.
What I suspect you'll see is that the guide comes up clearly while the small TV picture is doing its pixellating thing. What that tells you is that the TV is OK, and that the problenm is being caused by a poor cable signal coming into the cable box.
And if that's what's happening, you need to have your cable company come out and troubleshoot this for you.
Let me know if you have any other questions, and thanks for using JustAnswer!