The parade of issues and bugs in recent versions of Ubuntu is becoming a nuisance. The drivers B43 and STA for the Broadcom wifi chipsets popular in notebooks have a particularly irritating and serious bug: when on battery, the wifi chipset toggles between the on and off state on a ms time scale even when a connection exists, resulting in a latency of several 100 ms and low transmission rates. Since my mini also has a Broadcom chipset:

cobra@mini ~> lspci | grep Network
03:00.0 Network controller: Broadcom Corporation BCM4312 802.11b/g LP-PHY (rev 01),

I've notived this problem when attempting to run applications from my desktop via ssh, such as 'ssh -Y -C mathematica'. It's unbearably slow.

A temporary solution is to simply switch off power management for the chip for the time being:

iwconfig wlan0 power off

A permanent one is established by editing /usr/lib/pm-utils/power.d/wireless and to ensure that all entries under case $driver concerning operation with battery are equal to the corresponding ones with ac power on (the above link has an explicit listing).