Hi..
My only idea is this VMware KB: NFS connectivity issues on NetApp NFS filers on ESXi 5.x .
NetApp Bug decription here: http://support.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=321428
So if not having OnTap 8.1.2P4 installed this might be the issue that you're seing...?
/Rubeck