Dr Planarium has it exactly correct. Modern video compression is "temporal", meaning that it mostly only transmits the differences between each frame, and occasionally sends a whole frame (the "keyframe"). When the whole picture is changing (for instance, if you see a shot of ripples on a body of water), the transmitter is basically sending continuous keyframes, and has to resort to "spacial" compression (removing image data frame-by-frame) to try to keep the bitrate down. The transmitter might also drop frames, making the motion jerkier.
If you have low signal strength, these periods of high bitrate can look worse. Do us a favour and use your TV's settings to check the signal strength. If it's under 60%, you should upgrade your aerial; but before you do that, simply change the RF cables from the wall to the TV. Often when people buy a new digital TV, they just keep using the worn-out, brittle, corroded RF cables that they used for 15 years, and you can imagine what that does to the signal. Replace the cabling and see if that makes an improvement.
If it does, then you've fixed the problem. If it doesn't, then upgrade the aerial and put some sort of filtering device in the signal path. Many surge protector boards from electrical retailers have RF plugs to protect against surges, and these can also resolve interference problems (if you have high signal strength).