Alot of devices use the same frequency range as your wireless network. This includes 2.4GHz cordless phones, bluetooth devices, microwave ovens, and of course other wireless networks. Have you any of these nearby? I can think of a couple of suggestions. First make sure that the AP and your WL adaptor has the latest drivers installed. I notice alot of issues are related to outdated drivers. Second instead of letting the AP automatically select the channel try manually setting it. The most busy channels are usually 1,6 and 11. Try channels other than those three. Third try changing the location of your AP to see if that makes any difference. Or you can try purchasing a high-gain antenna if your AP supports that.
On another note I had a G.E. 2.4GHz cordless phone that was constantly wreaking havoc with my 802.11G network. I have since replaced the phone with a 5.4GHz phone and no problems since.