Hi
Virgin are using exactly the same equipment as Telewest, all that has changed is the name.
Having said that, they have also rolled out the 20Mb service, which seems to have been sold without regard for the capacity of the infrastructure, resulting in some areas being oversubscribed.
Your problem 'sounds' like you live in an oversubscribed area, as if it was a problem with your hardware or the cabling one would expect it wouldn't work, at all?
The difficult thing is what to advise you to do next, as, unfortunately, only Virgin Media can address the problem of over subscription - and they will only do that if enough people complain....
<edit> You can call 0800 561 0061 to find out if there are specific problems in your area, but it may not flag up over subscription.