Ok.. Do you know how to interpretate the info from commands?..
Take a close look at it, at the iwconfig....
Let me quote:
wlan1 IEEE 802.11bg ESSID:"XXXX_XXX"
Mode:Managed Frequency:2.437 GHz Access Point: XX:XX:XX:XX:XX:XX
Bit Rate=54 Mb/s Tx-Power=20 dBm
Retry long limit:7 RTS thrff Fragment thrff
Link Quality=44/70 Signal level=-66 dBm
Rx invalid nwid:0 Rx invalid crypt:0 Rx invalid frag:0
Tx excessive retries:0 Invalid misc:0 Missed beacon:0
You can have 100% power on signal, BUT "Link Quality" will define that, LINK..You are little bit above 60%, and man, I've got many many time seeing that problem...Bad Link Quality, BAD conection, no matter the strenght...It means too much interference (RFI noise, walls, windows, any aluminum mesh in windows, bricks, etc...).
Get close to AP, the link quality says so.
If problem still even with a much better quality, (>= 85%), you might have a bad Alfa.