Hi @Hugo_b ,
Does the recording have the same pixelation issues at the same point if played back at a different time ?If so that suggests the issue is occurring at the time of recording.
Do you experience the problem with recordings made at a time when you are watching another live Freeview channel without problems.
Any pattern to the problems .. eg recordings that would have been made when the Youview box would have been in standby mode.
Also need to consider what potential interference to the aerial signal could be occurring at that time. Are these recordings being made whilst other devices connected to your TV are in use ? Also any other cables for devices that are run in close proximity to the aerial cable that might be in use at the time.