BT Question can anbody advise please?

Hi,Having problems my D/L speed (aren't we all) JUl 07 I went to Virgin.net and was quite happy obtaining 5.7 meg on the up to 8 meg virgin Service. Over the months it dropped to 4 then 3 meg and now it's down to 1 or 2 meg.At worst its slower than dial up.Without boring you with the frustrating conversations with the ISP I have got to the stage of the good old BT speedtester.So far in 2 days I have managed to get just one test completed.
The result was:- The IP profile for your line was 2000kbps., 448kbps upstream , 4160kbps downstream. Actual IP throughout was 1737kbps.
To a newbie my first question was why have I lost 2.4meg. Am I missing something?Also having tried to carry out further BT speedtests the first page (where you put your Tel no. in shows my IP profile to have changed next day to 3500kbps.How is this changing? Please bear in mind I have achieved 5.7me d/l early in the contract with the ISP.
Thanks for your time

The IP profile follows your sync speed, so if it drops from 4000 to 2000 it will have done this because your modem connected at around 2400Kbps at one point in the last few days.

http://www.thinkbroadband.com/faq/sections/radsl.html#228 may explain some more.