Could be a signal level issue that would need adjusting by an engineer but as you've got an old V+ box I'd be more inclined to suggest the tuners or the HDD are failing. As these boxes are no longer supplied, the replacement would be a TiVo or V6 with the latter only supplied if you also have broadband with Virgin.
You can either ring up 150 / 0345 454 1111 and select "if there's a fault with your TV service" to get them to run the remote diagnostics or you can wait for the forum team to do this but they will take around a week's time.
--------------------------
TV: XL TV + Sky Sports (V6 & V+ HD)
Broadband: VIVID 200
Phone: Talk Unlimited