There are 2 free utilities which we find can be quite helpful when diagnosing Wireless LAN (WLAN) issues.  Note that we do not provide support for these products.

Netstumbler searches all the wireless channels, and displays a list of all the Wireless Access Points (WAPs) within range of your computer.  Moreover it records a graph of signal strength for each WAP; which shows clearly  any variations in the signal strength. See Introduction to NetStumbler
Ethereal is used to capture and display the packets of data travelling along an Ethernet LAN cable; or through the WLAN.

Please also checkout’s Tutorials, including Troubleshooting Poor WLAN Performance