I'm having exactly the same problem since 14th May. Two engineers have been to my premises and haven't been able to fix the problem or identify the cause. I'm on CMCGF exchange.
Welcome to the community forums
I have started you a new post as your problem could hasve a different cause and to save confusion in replies
Here is a basic guide to getting help from the community members done by CL Keith Please read through the link posted http://forumhelp.dyndns.info/speed/first_steps.html
once you have posted the information asked for then the community members can help you more
This is a customer to customer self help forum the only BT presence here are the forum moderators