It is down to insufficient signal. It could be an aerial problem, or the connections/lead, or it could be the Freeview box. Given you have a separate set-top box I'm assuming the TV is probably not the cause, as the signal will have been changed from digital to analogue before it gets to the TV.
(Silly thought, your aerial is still fixed firmly pointing in the right direction ?)
The only way to tell is to check each link in the chain and see if you can improve the signal.
First off wriggle pull/reinsert all the plugs/sockets to ensure a good connection. If that fails you might want to try adding a signal booster (put in the connection as near to the aerial as practical). After that if there is still problems I'd recommend replacing parts of the chain one at a time, checking if the problem has been fixed at each stage.
Ultimately, if you are using the original analogue aerial for digital signal reception, it might pay you to get an aerial designed for the Freeview signal installed. But check/replace the cheaper parts of the system first, before going on to the more expensive parts.