Airport can't connect to network

Hi all- Looking for help with a problem connecting to my wireless
router.
I have a D-Link 713P wireless router and I am trying to connect to it
with a flat screen Imac with an Airport extreme card. The imac is
running full retail version of OS X Panther.
The Airport detects the router and gets a half-strength signal.
Internet Connect displays "Getting status.." and then the signal
disappears. This cycles indefinitely. I can get some data in between
drops. The Imac can connect to same router via ethernet, and can
connect to neighbors wireless router across the street (a linksys
router). All connection controls (MAC,WEP etc) are turned off on the
router and I upgraded to latest firmware.
Any ideas?
Thanks