Is your box connected via Ethernet?
Have you set the box back to factory settings?
Are you talking about broadcast TV from the Smart Box?
Is catchup on the Smart Box OK, or does that pixelate also?
If, as a test, you unplug the aerial from the Smart Box, plug it directly into the aerial socket on the TV, tune the TV, and then try to watch a broadcast programme on the same channel that would pixelate on the Smart Box, is it OK directly on the TV, or does it still pixelate?