Timeout problems after enabling LDAP

We have configured an AIX 5.2 system to as an LDAP client to OpenLDAP (running on OS X 10.5 Server). Now many operations hang for approximately 75 seconds before completing. Anything that needs to map user/group id to string does this (for example ls -l) and it appears creating server side sockets does this too if the process is owned by a user held in the LDAP database. However the system is talking to the OpenLDAP server and can resolve usernames/groups correctly.

Anyone got a suggestion on what's going on and how to get it resolved?

Thanks
Mark