NFS not reflecting file changes.

I need help tracking down a problem. We occasionally
(every few days) have situations where changes to files
on an NFS-mounted partition that are done by one client
are not visible by a 2nd client - until the 2nd client
umount/mount cycles the nfs partition. The 2nd client
shows in dmesg:

NFS: Buggy server - nlink == 0!
__nfs_fhget: iget failed

The both clients might be any of a number of machines.

Server is a dual-athlon 1800+ box with 1.5GB ram.
Most clients (including the ones involved above)
are identical boxes to the server and connected via
a switched gigabit ethernet. showmount on the server
shows 30 external nfs mounts (some of which may be
duplicated, as showmount lists some mounts twice:
once by ip-address and one by domainname).

Kernel (all machines):
->uname -a
Linux XXX.XXX.XXX.XXX 2.4.18-24.8.0smp #1 SMP Fri Jan \
31 07:12:12EST 2003 i686 athlon i386 GNU/Linux

NFS-utils (all machines):
->rpm -qa | grep nfs
nfs-utils-1.0.5-2

Does anyone know what might be happening? More importantly,
does anyone have a fix?

Thanks in advance!
Steve