After upgrading a customer's ADSL connection from 512Kbps to 8Mbps, the throughput of the connection has dropped to around 100Kbps.
NOTE this is NOT the initial stabilising period - the line was upgraded 6 weeks ago.
The router is consistently syncing around 5Mbps, but I can only achieve around 100Kbps download speeds. I've tried different ADSL modems (which have been tested at other locations - D-Link, Netgear, 3Com and a few other Conexant ones), but no difference.
Everything else is unplugged for the tests - just laptop, ADSL modem, wall socket...and no, nothing wrong with the laptop...no spyware or viruses that are swamping the bandwidth.
BT have sent an engineer round on 3 occassions. Each time they plug in their equipment and report there's no problem. On one occassion after they left the line did speed up to around 4Mbps, but then next day is was back to 100Kbps.
As far as BT is concerned, there is no problem between the site and the exchange. And the broadband supplier says there's nothing wrong at their end.
What can I try next?