Is it the DSL G604T? There could be a number of reasons. Who is your ISP? When does it "go off"?
If the wireless signal drops suddenly whilst you`re using it it could be interference. For example someone using a microwave oven. DECT telephones also cause interference - do you have a base station next to the router?
If there is another wireless router within range using an identical channel or one close to yours this can also cause you to lose your signal. I`ve also noticed an increase in dropped lines, notably first thing in the morning. BT inform me that because there are so many exchange upgrades being undertaken overnight that this is to be expected.