Did you also check /var/log/messages on the nfs server side ?
I had some NFS troubles with lockd some times ago and it was a firewall problem on the client:
Try: - log on the NFS server and check in /var/log/messages which client is responsible for the problem (it could be an other one than your client). - on this client stop iptables (service iptables stop) and check if the problem still exist.
In my configs, client iptables fully trust my NFS server.
Patrick