HELP: NFS mount hangs when attempting to copy file

[Timothy Miller <theosib@gmail.com>]

My research suggests that NFS client mounting is kernel-based, so
that's why I'm posting here.  If there's a more appropriate list to
post to, I apologise, but I am not a list member.

I'm having a bit of a problem doing simple copies over an NFS mount. 
The client is running Linux (2.6.11), and the server is running
Solaris (5.8).

When I first boot the client, getting NFS directory listings works
just fine.  But the instant I try to copy a file (to or from), the NFS
mount hangs.  While I can still do other network activity (even rlogin
to the server), any NFS access I try to do after that point hangs,
including directory listings.

I have had this same client and server working flawlessly for years. 
The only change is that the client is now on a VPN (Watchguard SOHO
box).  However, I have a Sun machine on the same VPN network segment,
and it can copy files with no problem, so it's not the router/SOHO
that's blocking anything.  (NIS and DNS also work just fine for both
machines.)

Also, after it hangs like that, I cannot reboot the machine normally. 
When attempting to unmount the network filesystems, the shutdown
hangs, and I have to hard-reset the machine.
 
Is there anyone who could please help me to debug this problem?  As
far as I know, I have NFS setup properly, but I don't know enough
about it to know what options I might try.  I don't even care if the
fix degrades performance; I just want it to not hang.

Does anyone have any ideas? 
 
Thanks very much in advance!