Some months ago, Mark Lord posted a screenshot of a Linux-based ADSL diagnostic tool that gave a lot of information about the ADSL connection and, if my memory serves me, line quality etc.
I've been looking for that thread for the past hour and I can't find it (I had meant to bookmark it) - does anyone have a link (please)?

A friend of mine has recently upgraded from 2MB to 8MB ADSL. He is experiencing instability problems so I'm planning on putting a rig together and testing his connection. Any pointers or advice on how to diagnose ADSL line problems? He was perfectly stable at 2MB. I've told him to replace the filter and unplug everything other than the modem for diagnostic purposes.