environment: NW65sp6, client: 4.91sp3 & 4.91sp2 on XP

Some users complain that their files cannot be modified (rename, modify,
delete...)... and then things are okay after, say, 10 minutes. The
system doesn't seem to release the file lock...

.... after a few weeks, from time to time, files browsed with Windows
explorer are locked.

Because many workstations have Windows XP automatic update turned on... I
wonder if the recent patches from Microsoft causes the problem.

I turned off "file caching" and turned on "file commit" on the client
settings... could that be the problem?