I’m so sorry for not answeringbut it seems that problem was (is) in signal strenght. When signal strenght is over 70% there’s no problem with connection.
I’m expiriencing the same problem with Ubuntu, Backtrack etc.
I think You might get better help in the forums wireless subsection. Generally as far as I know the driver controls the power management of a wireless NIC so it may be a problem with the driver. Sometimes Windows drivers are better so You could try testing this using Windows and see if that helps (of course if You own one :P). Do not use ndiswrapper because You will always get worse results with native linux drivers.
Other than that a lot of devices can cause interference in the WiFi radio band and it’s usually impossible to tell measure that interference without some specialistic equipement. You could also try switching the WiFi channel on your access point so that You reduce interference from other access points if they are around and are using the same channle. I’m no expert on any of the topics I mentioned so please consult the wireless forums for better help