Faults is free on 151 from a VM landline or local-rate 0845 454 1111 from any other - note not premium rate.
We need more information about your setup; do you have a router, what OS, what happens when you connect your PC directly to the modem, have you tried the optimisations (
http://www.virginmedia.com/help/20mb...timisation.php).
Further to that I'm sure someone with more technical knowledge will be able to analyse your signal levels.