Try manually associating yourself with an AP:
ifconfig <iface> down # just to make sure
ifconfig <iface> up
iwconfig <iface> ap <mac-of-ap>
iwconfig <iface> key .....
run iwconfig <iface> a few times with a few seconds delay.
See if your wireless car associated with the AP, check transfer speed, see if frequency remains constant
If the card associated with the AP and stays that way, check ifconfig <iface>
if no IP associated, run dhcpcd <iface> or dhclient <iface> or assign an IP manually
Check route, add default gateway if not there
Check /etc/resolve.conf for primary dns name (usually "nameserver <IP of GW>" works)
Thats all I can think of now.
Please repply if you need help.