My guess is that it's either local radio frequency interference or interference coming through the mains power line that's causing the problem.
If either you or a neighbour have your central heating set to go off at 9.00am when you go out to work, and come on a gain at 6.00pm when you return from work (with a timer clock that's 11 minutes slow), that could easily be enough to cause your Freeview box to throw a wobbly.
If the problem is radio frequency interference, rather than mains-borne, fitting a hi-pass filter in between your aerial lead and the Freeview box is likely to solve the problem:
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