Bizarre speed issue

Apologies for cross posting here but I don't think the peeps in my ISP forum can help.

I signed up to Namesco 8Mb and I have been connected for a few days now.
The BT checker said I could get 6.5Mb and when I was connected, I was downloading at about 1Mb speeds which increased a couple of days later to 3Mb speeds and then falling to 512Kb speeds for a few days. Today I did a speedtest at http://test.speedtester.bt.com:50301/ and the results are posted below:


Test1 comprises of Best Effort Test:
IP profile for your line is - 7150 kbps
DSL connection rate: 448 kbps(UP-STREAM) 8128 kbps(DOWN-STREAM)
Actual IP throughput achieved during the test was - 5584 kbps


AG Speedtester result is here:
http://www.adslguide.org.uk/tools/speedchart.asp?id=11587700607414394401

Namesco speedtester says I am currently connected at 3Mb speeds.
Since my connection went live, my SNR has been between 12 and 15 (currently 12) and my margin has been a steady 13.0

I expected that my speeds would fluctuate somewhat during the training period but I am a little confused by the variety of different results for speed tests using AG, Namesco and BT. Are my SNR and Margin figures good for more than the 6.5Mb speeds quoted by BT?

If it helps, I am using a Draytek 2800VG and have also used the Namesco supplied Zyxel modem to eliminate any possible Draytek related problems.

*** Update: I tried tweaking my MTU and immediately my ISP's speed tester shot up to 6Mb but a few seconds later, I was back to 3Mb speeds.

Pulling my hair out here and desperate for some help.

TIA

Your actual speed will likely vary considerably as to time of day due to it being a contended product, i.e. you share the available bandwidht with others and the more online and downloading the less to go round. Speed testers should only be looked on as aguide at best and you are likely, all else being equal, seeing the best speeds at less busy times. Your proflle from the BT tester shows that you are on the highest speed profile and the 5xxx speeds you actually got during the test aren't too bad considering. So the range of speeds from the various speed testers would appear to be purely down contention which could by anywhere between you and the site you are connecting to. I.e. you to the exchange, the exchange to your ISP, your ISPs internal network and then out to wherever you are conecting to or any combination of the above.