Hey Guys,
I presume its the DMZ that caused the issue, although I'm not 100% on that, just with the timing of the emails and letters i'd recieved after doing so.
I did add port 5353 to my PC's firewall to be blocked, whether that helped or not I'm not sure.
I haven't tried the suggested Port Forwarding on the superhub yet as its now back in Modem mode. It might be something I'll try eventually though.
@Exe_uz My Line quality is pretty spikey also, it's not so bad when I've got my sliders set to 75% down and 30% up. (200d/20u)