Wireless Adapter Shuts Itself Off

I have Windows Vista Business on a Lenovo T60, Core 2 Duo T7700, 2GB RAM and
Intel PRO Wireless 3945abg network adapter.

Everything runs fine, until I step away from the computer. I have noticed
that after a certain time of no network activity, the network drops out &
the computer refuses to connect to any Wireless Network and just pops out a
"Could not connect to this Wireless Network" message (sometimes immediately
after selecting my network in the 'Connect or Disconnect' box). Diagnose &
Repair also doesnt do any good and asks me to contact the wireless network
admin. On some rare occassions this has happened when I was actively working
on th network as well. The only way to get WiFi back up and running is
1. Reboot the system
2. Disable and Enable the Wireless Adapter in Device Manager

I have begun to leave Internet Radio running in mute all night nowadays just
to ensure that WiFi stays connected all the time. I have opted out of "Allow
Windows to shut this device off" for the network adapter in the Device
Manager. No dough...

What could be the problem here?

Many thanks