Thank you for all your advices.
I have a similar problem with a brand new Lenovo ThinkPad Twist i7 S230U: uneven slow speed and frequent disconnections.
After trying all the recommendations I do not have disconnections anymore, but the speed is still inconstant, waving from 0,1 Mbps to 8 Mbps. (I have other laptops connected to the same WiFi and the speed is stable around 10Mbps)
Is Intel offering a solution to this commonly reported problem with the N2230 runing in Windows 8, or should be blame Lenovo?
Thank You