Ever since Virgin Media upgraded my cable internet to 2Mbps in November I have been having a problem where when I initiate multiple downloads the line drops out every 5 minutes. I had to call them on their premium line for cable modem support and they admitted there was a problem with the modem, and they refunded my call! They do refund the call if they admit is their problem. They sent me an installer to repair the modem and when he arrived he told me that the cable modem I had, Terayton, is not compatible with their service any more and he replaced the modem with a really minuscule one. Now the line seems working again :-)
So if you have a Terayton cable modem with your virgin media service its worth asking them to have a look.