Dodgy tuner on dvd freeview recorder ?

The tuner on my wharfedale dvd freeview recorder keeps losing the channels and just flicks to a black screen.
sometimes the channel is showing ok but it can flick to a black screen in an instant, so I never know if my recorded selections will be ok or black.
Im based in central london, so dont think its a signal reception issue, as have not had problems with others.
any help or suggestions much appreciated