NFS over UDP will hang under load and throw the following errors to syslog:
May  8 14:36:55 nfsclient05 kernel: impossible packet length (13050202) from nfs server 192.168.101.12:/store22
May  8 14:36:56 nfsclient05 kernel: impossible packet length (1669907873) from nfs server 192.168.101.12:/store22
May  8 14:37:01 nfsclient05 kernel: impossible packet length (46381) from nfs server 192.168.101.12:/store22

I have tried many mount options, but here is one set that will have this problem:
192.168.101.12:/store22       /s22           nfs     ro,-i,-b,-3,-T,-r=16384,-w=16384 0 0

To get the error to occur, I simply copy a multi-gig file from NFS to a local drive. It happens very frequently.  I have only tried this on gigabit networks.

The error goes away if I use UDP, which is what I am using until this is fixed.

I have tried this on FreeBSD 6.1 as well and had the same problem. I have also tried a different NFS server running a different OS on a different network with a different configuration on different hardware. It did not help. I have tried updating with freebsd-update but the status is unchanged.

Fix: 

Use UDP:
192.168.101.12:/store22       /s22           nfs     ro,-i,-b,-3,-U,-r=16384,-w=16384 0 0
How-To-Repeat: Put an NFS client and server on a gigabit network. Mount the server using TCP:
192.168.101.12:/store22       /s22           nfs     ro,-i,-b,-3,-T,-r=16384,-w=16384 0 0

Copy a large file (1GB+) from the NFS mount to a local drive. Do this a couple of times, then check /var/log/messages