My Friends Internet Is Not Working Properly. Does Anyone Know How To Fix It?

they just got at&t dsl at the beginning of the year and in February i helped set up a linksys router. at first it didnt work because the ip address of the router was the same as the ip address of the modem. but that was fixed by an att tech. it worked fine until about a week or so ago. then it stopped working completely for all devices. the power, and ethernet lights would be on, the dsl light would be blinking green. the internet and activity lights would be off. i tried to find out what was wrong and was unsuccessful. so they called the tech who couldnt figure it out so they gave them a new modem. it worked fine for about a day then the same thing happened. my friends mom doesnt know much about technology so the started blameing my friend playing xbox and his games getting viruses everywhere which is insane and impossible. n e way he got fed up and disconnected the router and connected the computer directly to the modem again. and a few hours later the modem magically started working again. I KNOW for a fact that it is not the xbox but is it the router? can it be the ip address thing again? any other ideas? i want to get this working for my friend so he can play xbox on live again. but i am kind of stumped. i think it is still at&t effin up again because i have had the same service for years and pretty much the same router for 6 months longer than him and had no problems like that.
the modem is not a router. and the puzzling thing is that it worked fine for months before it suddenly started working. and isnt the dsl light have to do with its connection through the phone line and not the connection of the router?