Wireless :: Signals Die Out Completely Every Few Minutes?

For quite some time now my parents' wireless internet cannot seem to keep a wireless signal anywhere in the house unless you are sitting next to the router. I've been doing everything I know to try and keep the signal strong enough to use it down the hall in the living room.

I've tried using different frequencies and channels however this only seems to work for an hour then the network will fall off the network list entirely and then a few minutes later will pop back in. What is causing this signal to die out completely every few minutes? They've tried countless numbers of wireless routers in hopes it would fix the problem however this never seems to work. Is this an issue with the ISP and/or the modem they provide us? Wired connections work fine, just wireless connections refuse to either stay up or constantly disconnect the user from the network.