You might want to take the PC to the master socket, remove the "extensions"
faceplate (the bit you're allowed to remove and then plug in the PC direftly
into the master socket. This isolates you from any nasties where someone
has added the extensions around the house. If you still get poor quality,
it might be time to talk to BT about their wiring.
Alternatively, if all now works well, you will need to sort out the badly
wired extensions.
If you don't know what I'm talking about here, try looking at the
http://www.clarify.it website for a while. They have lots of pictures of
the NET5 (master socket) on there.
Paul DS.