It could be this: [bugs.launchpad.net ]
There is a possible fix suggested in the last comment. You can test it by seeing if:
sudo sysctl -w dev.wifi0.intmit=0
makes a difference, and then change the config file if it does.
It could also be a network manager problem. You could try Wicd, but it does not support mobile (i.e. cellular), two interfaces at once (i.e. ethernet and wifi both connected at the same time) and a few other things (including, I think, dialup and USB modem): [wicd.sourceforge.net ]
Please note, installing Wicd will remove Network manager.