RE: Virgin media superhub
Not familiar with these virgin superhubs, but I gather it is a combination modem/router -- if this device is normally operating in NAT mode, see if you can place it in bridge mode, thus giving the fgt an " outside" public IP address.
Getting back to your problem, I would suspect a possible duplex/speed mismatch between the fgt and superhub -- you can check for this on the fgt side by issuing the following statement on the CLI-> " diag hardware deviceinfo nic <interface port>" where <interface port" is the port you have the superhub connected to. On the otherside, if the superhub has an diag page you can access, I would check that as well. You will be looking for any kind of bit or transmission errors. Alternately, if debugging/troubleshooting is very limited you could try forcing the connection speed on the WAN interface of the fgt.
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C