You are probably getting interference. Are there any other networks in the area?
Try logging into your router and changing it to a channel different from a network that may be interfering with yours.
To log into your router go to http://192.168.1.1/
and enter your login information. If you don't know the default login information, check this site
Channels 1, 6, and 11 are what you want to use. The channels in between those overlap.
Also go here
to see what the wireless spectrum is like in my house.