There are a number of possible causes of picture break-up, pixelation or freezing:
If you are in an area where the signal strength is low, you may need a high-gain aerial for the best reception.
Your aerial should ideally be mounted outside and as high up as possible. It must be pointing in the right direction towards the best local transmitter
The aerial, cables and connections need to be in good condition - cables and connections in particular can become corroded, especially if you live near the sea
It's possible that outside objects are effecting your DVB-T reception , especially if there are large trees near your TV aerial
The TV signal travels in a straight line from the transmitter to the aerial and does not bend well around large objects such as trees. The signal may fluctuate in wet or windy weather. Trees can act as a barrier to the signal, resulting the picture to break up or freeze