so it sounds like an IPv6 problem. If you already know stop me
IPv6 is the new version of IP address (eg 192.168.1.1). IPv6 will allow 'everyone' to have their own IP address. It's also suppose to be faster. To be honest IPv6 has been around in theory for a while, I heard of it I think 4 years ago, but it's not really caught yet. The net is still happy with IPv4.
Why exactly it works better without IPv6? Modem dosen't support it? translation of IPv6 to IPv4 is bad, your DSL provider doesn't like IPv6?
Don't know exactly how you would find out which is the one.