With digital TV the source is either good enough for a picture or not. they don't get fuzzy or noisy like old analog sets. A very marginal signal would have the picture go on and off as you describe. Check the things that Jayeff described and if you use cable then have the company send someone to check the strength.
Some cable boxes/DVRs have a signal checker in the setup menu (Comcast X1 for example is under the Gear icon > Comcast Labs > Signal strength).