If it's the Freeview HD channels only then you need to check your local transmitter isn't having any issues, check your aerial connection is good and away from other cables behind the YV box/TV. What signal strength/quality do you get with the HD channels?
Have you checked for any engineering work in your area? http://www.digitaluk.co.uk/help_and_advice/engineering_works
it may also be 4G interference if a new mast has become operational in your area recently but that should affect all channels not just HD.