Try changing the channel number the router broadcasts on for wifi. See this more and more often lately as more neighbours get broadband they all seem to want to use channel 6 - which wipes out 5,6,7 as being any good, the signal will come and go. If you've got an android phone get wifi analyser lets you see all available AP's and what channel they are on and who has the best signal etc.
I love that app. I was installing Cisco Aironet wireless in a 'shared' office building with nothing else but small businesses. Every channel was saturated!. I picked 1-3 in the end as that was the least saturated and enabled wide band and boosted the gain :devil: