Re: Exchange-only lines?
Either radio interference from dodgy electricals or poor connection on a terminal along the path. My ADSL was never amazing and VDSL sync'd at its 40Mbps cap but the line statistics were awful and the sync speed started dropping. I disconnected the bell wire, our unused internal extensions and made a new connection at the main socket. The result was close to double the SNR allowing continual max sync speed and a Fastpath rather than Interleaved connection. There is improvement to be had as periodic crackling and low level hum can be heard on the line - though I think that is outside the confines on our flat.