Just because I am out of ideas at the moment could you try disabling
IPv6 to see if that helps? You can do it without rebooting by (as root)
echo 1 > /proc/sys/net/ipv6/conf/(name of interface)/disable_ipv6
And then see if this makes any difference.
in fact, some time ago, I contacted my ISP to see if my issue could
come from their DNS and they asked me to change the default DNS and I
also set the "IPv4 required for this connexion". So I guess I am
already fully IPv4?
Frédéric