Why does Wireless Connections drop when using a secured network?

Hello,

This issue has been bothering me for awhile now.
My wireless NIC, Intel Pro 3945 ABG, is able to scan networks and connect to them.

It can maintain a stable connection when the network is unsecured. I can browse the internet for days without losing connection.
However, the connection to a secured network, WEP/WPA/WPA2, will drop within the first 1~2 minutes of accessing the internet. I can reconnect almost immediately after the connection drop, but it would repeat itself within 1~2 minutes.

It has been tested on both the drivers Toshiba provided and the latest Intel drivers, and I was able to reproduce the same results.

Can an expert point me in the right direction?

Forgot to mention, I am on Vista 32bit Enterprise.

Thanks in advance.