NFS mount hangs

[Norman Elton]

I've got a handful (about 20) client machines that mount a series of  
directories off of a server. In this case, the clients are RHEL4  
systems, and the server is a Mac OS Server. I use the automounter to  
automatically manage the mounts. The user merely cd's to the correct  
directory, and it automagically mounts itself.

99% of the time, this works flawlessly. There are times; however,  
that the mount process hangs. After upgrading the client RHEL systems  
to the latest kernel, its happened twice. In both cases, I can mount  
some NFS directories, while others hang. Even if I try to mount them  
manually, outside the context of the automounter, nothing happens.  
Meanwhile, other directories on the same client work fine.

In the past, the only way I can find to "fix" the problem is to  
reboot the client. This is obviously not ideal.

Here's a sample mount showing my config:

10.1.100.6:/Volumes/NetMacRAID/Shared/home on /mac/home type nfs  
(rw,addr=10.1.100.6)

I suspect that I don't have NFS configured/tuned very well. Does  
anyone have any suggestions here? Any idea why some exports  
mysteriously die?

Thanks for any help,

Norman