As you've stated it works fine in modem mode (which would kind of rule out the network card itself) & you've tried other devices which drop when in router mode via ethernet, then I'd probably buy a new router as Legacy1 suggested & leave it in modem mode, or you can always report it faulty & ask for a replacement hub & see if a new hub sorts it out & saves you a few quid first?
having just joined VM today, first thing i did was put it in modem only mode. Should VM replace it if its dodgy. When i put mine in modem only mode i used port 1. What if his port 1 is causing issues? Putting it in modem only mode might not fix it?