Perhaps signal interference (channel overlapping) from another (neighbor's) WiFi? You can use a tool such as inSSIDer (no longer free, google for older version 3 which is free) or Network Stumbler (doesn't work for me but you can try it) to verify if that is the case. There are also many free WiFi analyzer apps for Android and iOS which are also easier to use.
If you don't want to bother with verification, you can just blindly try to change WiFi/WLAN channel of your router (consult your router manuals) to some which is hopefully "free" (WLAN channels 1, 6 and 11 should be tried first).