Do you have neighnours living close to you? it is possible if you do that they have set up wifi on a conflicting channel to yours. there is an android app called wifi analyser that would help you discover this, im sure iphones have a similar app too. if you do have this problem then you will need to adjust your routers settings to put your wifi onto a different channel, normally quite easy to achieve
due to the nature of radio waves, you may only suffer this in certain areas
Edited by Fozzies, 05 October 2011 - 11:01 AM.