It's not something I have heard of to have such a drop that much, please check the phone line to make sure there is no noise or anything there you can check it by ringing 17070 and selecting quiet line test.
Once your confident this not the cause then run a speed test on <a href="http://speedtest.btwholesale.com">BT Speed Test</a> once the first test is complete click run further diagnostics enter your phone number and then submit the results to the server.
Then <a href="http://faults.plus.net>Raise a fault online</a> and one of my colleagues will pick this up within 72 working hours for you.
