Set a static on the client desktops, check both Ethernet and Wireless Nic’s... release dhcp using the command prompt.
“Click Start->Run, type cmd and press Enter. Type ipconfig /release at the prompt window, press Enter, it will release the current IP configuration. Type ipconfig /renew at the prompt window, press Enter, wait for a while, the DHCP server will assign a new IP address for your computer.”
Yes, this is clearly a DHCP issue. You may want to look over the settings to see if you can "reserve" the AP IP with its MAC address. That way that particular IP will only handed out to the AP. You can do the same with desktops, too. It would seem that the AP may behaving as BOOTP, where it does not renew its lease before expiration. Thus, the DHCP is giving that IP to a new requester.