It could be interference from nearby electrical/radio equipment. Cordless telephones, bluetooth devices, satellite dishes, microwave ovens, metallic items, remote garage door-openers and other wireless LANs using the same channel as you, can all present problems.
Changing the channel number in the router sometimes helps with connection and speed problems. Channels 1 and 11 are usually the best, but check who, locally, is using the same channel as you. In a Run window (Windows key+R), type cmd /k netsh wlan show networks mode=bssid (note the six spaces) and press Enter. Or for a nice graphical display download, install and run inSSIDer, click here Simply select your wireless adapter from the drop-down menu and click Start Scanning.