I have been with Pipex for a year now. I had that trouble at first. Not sure if it is the same thing but I forgot to get a microfilter for my Sky digibox. As soon as I unplugged the phone connection from the digibox my ADSL connection was fine. I since got a microfilter!
I e-mailed them about a problem recently and it took them a while to respond. They sent me a link to adslreports.com about making a tweak for BT lines. Since doing it my connection speed has been rocketing but I am not sure if that is just coincidence.