I had the same problem a couple of weeks ago, quite bad pixillation and sound dropping in and out, anyway rang VM and they sent an engineer out, that day!! great service.
Luckily the pixillation was occurring while the engineer was there, so he tested the signal strength from the green box, that was fine, the problem apparently was that I was losing signal strength between the outside drop-box and the connection inside the house.
Anyway long story short the guy replaced the white cable between said boxes and the problem was sorted...a truly great bit of customer service.
