NFS 'hang'

Via a tunnel interface I have a NFS share. FreeBSD 4.11 as server, linux
2.6.20 as client. This works quite well.

But once in a while a file get completely locked up. The client sends
READ requests, the server returns reqeust and the client marks them as
duplicate RPC Read calls :-/ I've made a dump with wireshark:

http://www.searchy.net/test/wireshark-nfs-dump.pcap

What can cause this?
Could it be prevented by connecting with tcp to the NFS server?

(Ps, a reset of the NFS server has no effect)