Locking records problems, how to solve it


I am getting a rare locking exception, I have a database over MS SQL 7.0
and my App was developed with delphi 4.
In a module of inventory there are some tables that are related, when I
want to change some information from a table I get the "Could not perform
edit because another user changed the record" exception, but the rare thing
is that only with one table, if I change any other data I do not get the
Most rare is that this exception raise only with some records and not with
all the records in the table.

The relation in table is  article -----  unit measure

I think this info can help you to help me.

Thanks in advance

Roberto Martinez