Do you have a firewall that drops ICMP packets? Some firewall admins do this believing it improves security, but it can cause exactly this to happen. You can u

Do you have a firewall that drops ICMP packets? Some firewall admins do this believing it improves security, but it can cause exactly this to happen. You can usually get away with it in IPv4, but with IPv6 it causes connections to stall as soon as you try to push any significant amount of data through a TCP connection.