Wireless network card spontaneously disconnects every 5 minutes: Why?

[RickJay]

I'm on my wife's laptop, a Dell Inspiron 640m, and for awhile now it's done exactly what the subject line describes; every five minutes or so, it simply drops the network connection. It happens with all wireless networks, all the time. It happens whether or not the AC adapter is plugged in. It happens with all applications. Hardware settings suggest no problem.

Once it disconnects, within a few seconds you can reconnect. So I can post this, but it sure is irritating.

The ethernet port does't do this. Works perfectly.

Has anyone seen this problem before or know how it might be fixed? Note: It's a built-in wireless card, not something that can be swapped out.