ring up and ask to be put through to retentions, say you're really unhappy and that unless they give you x amount of money off your bills you're leaving, and if they dont sort it out you will be leaving too.
I had this same issue for a few months, they basically admitted to me that they had too many people on the same line but that they were laying a new line and it would be sorted after that, which it was.
Must say, i've had virgin media on and off for a total of about 3 years, only had 1 bad experience with them though