I have problems with Network Manager at certain locations that use repeaters or multiple access points with the same SSID. At one library in particular, I see similar behaviour to what you describe. I have a very simple wifi tool called "rutilt" installed on my laptop and it will connect at the same library OK, but it is necessary to unload NM first. You may have better luck with wifi-radar if you do the same. (How did you install wifi-radar? I believe if you use the ubuntu repositories installing wifi-radar forces NM to uninstall)
There is another network manager-like program named wicd that many people use. It is more of a replacement for Network Manager. I tried it about a year ago, it worked OK but not perfectly for me. Perhaps the latest version has improved.
Anyway, the exact commands to unload Network Manager are slightly different depending on what version you are running. In ubuntu 8.04 it could be unloaded with
sudo /etc/dbus-1/event.d/25NetworkManager stop
sudo /etc/dbus-1/event.d/26NetworkManagerDispatcher stop
In 9.04 (I don't remember what worked in 8.10) it is just
sudo /etc/init.d/NetworkManager stop
To restart NM run the same commands but replace "stop" with "start"