I've been having the same issues for a week now and phoning BT just to be told that my line is fine. I've been running a program today called pingplotter (it saves doing constant manual traceroutes) and the results it shows for the BT network are quite astounding. I've tried getting through to BT about this again but as soon as I get to customer service they just insist my line has no issues. When I ask about what network problems they are experiencing they have no answer as it's not in their script. Worst service I have had in my 20 years of internet usage.
