While I was able to get the maximum out of my 8Mbps connection, 7150 profile, in normal use, I was getting a similar situation with regards to speed tests on this site, and some others, compared to download tests and the BT speedtester site.
If I used my download manager to download a test file from here, I could get an average of 850 KBytes a second download, the tester would give a maximum of 200 KBytes a second, the BT Speedtester site was in the 600 to 700 KBytes a second range.
My download manager allowed me to make multiple connections for the download, so I restricted it to a single connection, result was that maximum throughput on the download was about 200 KBytes a second.
This meant that the sites which were showing slow results used a single connection for the test, where I got a high result there were multiple connections, my slow down was due to a single connection being throttled to about 200 KBytes a second, but where was the throttle ?
I tried a different router, I usually use a customised IPCop build, same issue on Voyager 205 router.
That left the local machine or the ISP side being the problem. I did some checking into the type of issue and found the TCP/IP analyser at http://www.speedguide.net/analyzer.php
This told me that the maximum throughput the machine would allow is about 200 KBytes a second due to the TCP Receive Window (RWIN) settings within Windows. There was info on solving this, and they also have a tool to download that allows you to tweak from an interface, rather than have to hand edit registry entries. I have set this to optimal, according to the site, settings for my connection, and now the speed test on this site, a single connection download test and the BT Speedtester all give similar results, 800 KBytes a second or so, when done at the same time of day.
May be that the buffering you are getting is due to single connections being in use and the machines current own limit for a single connection being reached. It might be worth trying this test to see if it is possible.