Had a major connection problem but eventually it was discovered that my neighbour had a massive satellite dish and the set-top box was interfering with everyone around, (ADSL), as soon as he turned it off we all got a good connection back. It is possible that someone in your area has electronic equipment that comes on an certain times and destroys your DSL signal with 'noise', worth investigating? (It helps if your ISP also owns the infrastructure, i.e. BT in the UK, Telstra in Australia etc. otherwise the companies that buy their connection from the major supplier are fairly helpless and the owner of the infrastructure will tell you that their responsibility ends at your front gate!).