If you could confirm that the TV does this from every source, or just the off air tuner. If it is just the tuner, you have poor signal quality on your incoming signal. It could be that one or more of the connections are made poorly, that the lead in cable is too long or damaged, or some other fault. There is likely to be a signal strength indicator built into your set, as part of the tuning and setup procedure. It might be helpful in finding the trouble.
Digital TV signals do not work like analog ones did, where the picture gets worse and worse until its unwatchable. Digital TV looks just great, until it doesn't, breaks up, and drops out. A borderline signal will drop out, and come back up, over and over again. Often with periods of broken up, pixelated frames in between.