If you're using the SuperHub 2, there's a known incompatibility which will cause random disconnections from Xbox Live.
To rectify this, Virgin Media will need to send you the SuperHub 2ac - a lot of customers have been receiving this for free, to help resolv e the issues for customers. A staff member should be able to arrange this for you.
Thanks for your reply, I've been able to have a look into this for you. We are currently investigating disconnections while playing online linked to a recent firmware update on the Superhub2. We should have some information regarding this very soon. We have been made aware that modem mode can be used as temporary workaround.