The daily variation is caused by radio frequency interference which is greater at night. Its something to do with the troposphere reflecting radio waves better at night, those of us old enough to remember the days of radio Luxemburg will know you could only get it at night! The ADSL signal is really a radio signal superimposed onto the wires which is why it still works with only one wire connected and the phone dosent. Hence at night when all those foreign radio signals are strongrer they interfere with the ADSL signal.