The RFX9600 utilizes the UDP protocol and there are known issues with UDP on Airports. I've seen this documented by other control vendors as well.
There is a secondary issue regarding security and the scheme you are using. WPA requires longer negotiation period for connection. So, the fact that you are using WPA coupled with Airport could be cause of many of your issues. Drop in a Linksys/Cisco/PakEdge WAP and run as WEP. That will likely correct many of your issues.
