Comment 14 for bug 105637

This bug looks like a combination of both madwifi and network-manager with neither being fully at fault!

Network manager scans for new networks whilst the card is associated with an access point.

It looks like madwifi doesn't support 'background scanning'

When a scan is performed on the instruction of network manager, the card disconnects from the access point.

See here for more info https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/37821

Fixes look to either prevent network manager from running scans whilst connected to an access point (after all, why would you want to?). Another fix would be to implement background scanning into the driver...