You may have heard some of the suggestions put forward here but a;
Check all your phones and make sure they each have a microfilter. Pick up a phone when the line is faulty and press a single digit once so you get a silent line - listen for interference, crackling, buzzing etc. Disconnect all other devices connected to the phone (including your Sky box if you have one) and see if the ADSL signal returns. Does it happen at precisely the same time every time, or is 5 o'clock an approximation? Do any of your neighbours have ADSL broadband and if so do they suffer the same problem?