I've been having mega problems with Virgin dropping me out, sometimes five times in as many minutes! They blame everything from BT gain to your own kit, etc.
The more cynical of us reckon they're deliberately doing it toforce people to step up to ADSL.
Try this Number, it's the one I use:-
You should contact your credit card issuer, and claw back the money temporarily. Then watch how fast they contact you! I did that with NTL a few years back, and boy, did they waken up then!
Good luck,; let me know how you get on?