It's possible that switching the Linux NFS server to a Netapp will help (which is what it will be deployed onto eventually anyway)NetApp should help, but I doubt it'll remove all the problems. Also Dovecot's NFS workarounds work better for Linux NFS client than for FreeBSD..
I doubt this is related to the specific problem, but I've had issues with FreeBSD clients when using NFS over UDP. You might try forcing tcp from them:
mount -o tcp host:/mount /mountpoint Rick
