Yes, good point made by throckenholt...
Can you get box to scan for channels ?
Town where I am is on edge of three regions, HTV, Central and Granada, and my old Freeview box picked up Central signals (poor) as they were on low channel numbers from 28 to 40 or so, while Granada (strong sig) channels are from 51 to 66, so I got Freeview box to scan for new channels but kept the aerial out until it had gone past the channels from Central, and got the stronger signals for Granada area.
With the changes coming over the next couple of years, there might have been some local (to your area) maintenance or changes. If re-scanning makes no difference, then a new box might be worthwhile.
With the lowest cost boxes at 15 or 20 quid in Asda, it may be worth getting a new box (cheaper than a repair) and seeing if that works better, and then keep your current box as a spare.
If you get the same problem, with a brand new box, then a few more details like the transmitter you get signals from (or TV region) might help (can search for transmitter news more easily)