Authentication issues

Hello all,

One of our users gets de-authenticated every few minutes from our
Cisco 1240AG APs. He is using a D-Link DWL-AG530 NIC.
I do have Cisco Aironet extensions enabled on the AP.

Any ideas will be appreciated.

Here is the Log entry from the AP
Mar 7 16:32:54.608 Information Interface Dot11Radio0, Station X
Associated KEY_MGMT[NONE]
Mar 7 16:32:52.305 Information Interface Dot11Radio0, Deauthenticating
Station X Reason: Previous authentication no longer valid
Mar 7 16:32:52.305 Warning Packet to client X reached max retries,
removing the client

Thanks.