How is the Xbox connected? Ethernet connection direct to the VM hub, Ethernet via powerline adapters or a wireless connection?
Disclaimer - I don't work for Virgin Media. I'm just another VM user trying to help out. My setup: V6 TV box, Vivid 200 Optical fibre with Hub 3 in modem mode connected to a Netgear R7000 router. Telewest/VM user since 2001.
-- Red (VM SH2 AC Beta tester) Running on 350Mb VM service with SH3 in Modem Mode, with a ASUS RT-AC5300 router. With this setup I get Fantastic WIRED and WIRELESS Signals in my home. I dont work for VM. But I work in IT, and I know my tech. My advice is at your own risk.