1. It is most likely caused by interference, the existence of “dead spots”, or poor reception that can be corrected by better locating/aligning an antenna.

2. The beacon strength of an access point is not a measure of the performance of your wireless network.  Nor do you want to necessarily associate with the access point with the strongest beacon signal.

3. In 802.11 b/g you have a choice of 14 channels (actually, 11 channels in North America) – where each channel is a range of frequencies.  There is tremendous overlap among adjacent channels (take a quick look at the image at the bottom of this page).  The reason why it’s important to make note of the overlap is that a wireless device that transmits in the frequency range of channel 1 not only interferes with channel 1 but probably also is wrecking havoc on channels 2, 3, etc…

4. When interference is the cause of poor performance, then the most common solution is to reconfigure your access point to use a different channel.  In certain situations you may need to track down the source of interference, but more often than not it will be coming from a source or location you have no control over.  So, the simplest "fix" is to identify a channel that isn’t subject to interference and reconfigure your wireless network to use that channel.