John, several folks have already suggested nfsstats -h. But you already know what clients are the problem. Here is what I suggest:
On each client, use lsof to find the processes with open NFS files. This reports on NFS files owned by user quentin, and excludes program binaries and directories:
lsof -N -a -u quentin | grep -vE 'txt|cwd'
For each process, stop it with `kill -STOP <pid>' If the load decreases you may have found your culprit.
Did the contents of the NFS traffic offer any hints? i.e. file ownership, file contents, etc?