Teredo tunneling pseudo-interface havoc
Teredo tunneling is a feature built into Windows (and other OSes) to allow the use of IPv6 over IPv4. I think it's supposed to work automatically, but it never did for me.
What it has been doing for me is causing havoc with my devices. When you run IPv6 natively it doesn't turn itself off, but just plods along trying to do IPv6 stuff too. The problem I had is with my firewalls not expecting the address range it used so I was getting all sorts of problems - specifically with my DNS server timing out.
A bit of packet sniffing later I realised where the requests were coming from. The solution is simple, albeit annoying that you have to do it at all. Fire-up an administrative command prompt and run the command:
netsh interface teredo set state disabled
Probably reboot for good measure, but that should solve a lot of problems!