Is it possible that you have the "hard" option set for the NFS mount? If so, there is no timeout, and any process wishing to access the mounted file system just hangs and waits until the file system is back again. And if a directory on that disk happens to be in your process' path, the process will just hang.
Set the soft option, and perhaps a timeout value in fstab (be careful, on a busy network you don't want the timeout to be too small)
machine.net.net:/share /share nfs rw,soft,bg,timout=10,nfsvers=3 0 0
Hope it helps,