I was having the same problem and after much searching I found a solution, I think it was on a kernel list.
I quote:-
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echo 0 > /proc/sys/net/ipv4/tcp_default_win_scale
or by adding a line like:
net.ipv4.tcp_default_win_scale = 0
to /etc/sysctl.conf.
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Apparently it actually due to a bug in some routers.