Comment 6 for bug 21471

The firmware solution fixed my problem as well. In fact, when the default firmware is loaded, the following message is put in /var/log/messages:

Jun 14 05:26:20 localhost kernel: [4323118.020000] acx: firmware 'Rev 2.3.1.31' does not work well with this driver

Ok, the firmware knows its incompatible, so why does Ubuntu use it?