I too am seeing this problem.
Using Centos 6 NFS.
Been fine when we were on 5.1 but recently upgraded to 5.5 U3 (3343343) and the problem has started.
The underlying server has never experienced APD over the last 2 years and only started since the upgrade.
I have put MaxQueueDepth to 64, same MTU across the board.
We have two NFS mounts from the same server and it only happens on the primary one which has active servers.
Does not occour during the day but normally at night when backups are running and does not seem related to underlying NFS storage as that is still available to all other servers when APD happens on a node.
It seems to happen at the end of the backup when the system is consolodating the VM it was working on.
The load on the underlying NFS server is low and I am not seeing major increased latency on other guest machines while the backup is happening, nothing that would not be expected during the working day.