You have 2 wireless cards : a BCM4306 and a BCM4318.
Wich one are you using ??? Are both cards actives ? Is the correct
driver loaded and associate with the correct NIC ? Could you show
output of 'iwconfig' and 'lsmod' ?

I'm writing this reply while connected to internet with a linksys
wireless-g card (version 03) wich is a BCM4318 chipset too. I have
used fwcutter to create a driver wich works at 11MBS. If you use the
windows driver with ndiswrapper you will get 54MBS. But I have not
succeeded installation with ndiswrapper yet ... I would prefer to
have 54 KBS since I'm connected with a fast internet connexion. I know
it's not "open source" ... but I'm only being practical : I want to
get all the speed I paid for the NIC... And it does matter for backups...

I can assure you that the connexion is perfectly stable and reliable.

To setup a driver with fwcutter I have followed instructions at the
following link:

http://ubuntuforums.org/showthread.php?t=185174

(Instructions are for edgy but are still OK with fiesty)

some usefull information also at :

https://help.ubuntu.com/community/WifiDocs/Driver/bcm43xx/Feisty

The only problem I had is that if you install the bcm43xx-fwcutter
package it doesn't install correctly and complains about a file it
doesn't find on a web site... I have deleted the package and
installed from the tarball and it stopped apt-get from complaining
about the unfinished installation ... Since it's a one time shot, I
don't really care about the package not being kept up to date with
apt-get. Be sure you have the correct driver (WK2000 or XP) on your
Desktop and follow instructions.