sub-optimal bandwidth

[markfoodyburton]

Hi,
Orange finally have sorted out two near identical ADSL lines for me, joy.
But I'm still not receiving the sum of the bandwidth.

Both lines are about 5Mbits/s
But, I peak out at around 7.5Mbits/s - 8Mbits/s.
The bandwidth as measured at the modems themselves is around the same amount (in other words, the overhead from MLVPN isn't the issue).
I've played with MTU, but it doesn't make much difference.
I've checked the traffic control on both ends, disabling any traffic control makes the bandwidth less stable, but does not give me any additional 'peak' bandwidth - it's not the traffic control that is limited my bandwidth.
(Anyway, this 'loss' is present no matter what my modem bandwidths are, so it is unlikely to be traffic control).

I am testing with e.g. iperf, (so there is no 'external' bandwidth issue, I am really only testing the tunnel) (though I have the same results with external data).

I have no idea what could cause this problem.

Does anybody else see anything similar?
Have you any ideas what could cause this?

Cheers
Mark.