Try:
- changing channels (using either 1, 6, or 11 is recommended)
- increasing the access point transmit power (if possible, don't go higher than 80mw on most consumer grade gear)
- disabling afterburner/speedbooster
- disabling CTS protection
- disabling frameburst
- forcing the preamble to long
- using WPA with only TKIP (as a last resort, TKIP is not as secure as AES, but is generally better supported)

Keep in mind this is a small device with a small 802.11 antenna - this will cause issues where you would not normally have them with other devices (that have higher gain antennas). Any interference (whether from other 802.11 or bluetooth, microwaves, cordless phones, etc) will also be more difficult to overpower, since there is less antenna gain and possibly less transmit power on the N900 end. In other words - if you are seeing this problem within a few feet of an access point it's most likely a software/configuration issue, but if you are much further away it's more likely a hardware limitation of a pocketable device.