This problem has seemed to appeared probably since the router was accidentally switched off along with the Virgin Modem at the plug, and despite a restart of both router and modem, adding Xbox to DMZ, opening port 3074, having Xbox go directly through a public DNS server as opposed to going through my local DNS server, unblocking UDP 443 and 53, nothing has worked.
The same thing seems to be happening on my brother's Xbox and for me the problem is disruptive enough as it causes me to be booted from GTA lobbies.
I think I''ll probably move onto another router that's more advanced, less buggy and cheap as well as this router has several unaddressed issues with one of them being the settings icon randomly showing a pause/play symbol and you can't access the settings page without restarting the thing. Pretty poor really.
However can anyone advise the possible issue in the meantime?