Vexing wireless IP address problem

We have three Dell laptops, all running WinXP Pro and current on patches, service packs, etc. Two are Inspiron 640m models; the other another Inspiron; all are runing the IntelProset/Wireless 3945ABG radio set and associated drivers.  If we power down the computers, move to another wireless network (eg., work to home or vice versa) and power up, the WiFi finds the local access points but searches for an IP address for about 30 secs, shows a popup with the LAST IP address used, and then quickly rolls over to the default autoconfig ip address and shows a windows ballon from the wireless icon in the system tray saying "Unable to Obtain an IP Address." If at this point, we reboot, the computers NOW can obtain an IP address. This problem does NOT occurr of the computer goes in an out of hibernate during the move between networks.

This is a daily problem taking the laptops between home and work and between meetings, so we've been leaving them in hibernate, but that's not a great fix b/c Windows doesn't hibertate well. Restarting twice works, but that's time consuming.

I've tried updating the Wifi driver, but that caused connectivity to fail entirely and had to roll back the driver. The problem also persisits if Windows manages the WiFi connection instead of the IntelProSet Wireless.

It appears that there is a communications problem between the WiFi driver and the DCHP or TCP/IP stack. The WiFi module finds the known or new access point and connects but does so in the wrong sequence with the DCHP issuance. If you reboot, and the WiFi knows which accesspoint it's looking for as it boots, the IP address can be issued. That's my current theory but I don't know how to fix it.