Unfortunately, the native rtl8187 module proves very unstable with the Rtl8187B: although the connection appears fine in the menu bar, the wifi interrupts its connection or is suddenly very slow.
After some investigation I found a workaround: http://ubuntuforums.org/showthread.php?t=792092
gksudo gedit /etc/rc.local
- add at the end, before the
iwconfig wlan0 rate 5.5M