DBGrid Freezing, Please Help

I have an application that has been in use for about 5 years.  It was
developed in Delphi 5 accessing an Oracle Database.  Recently we
upgraded to Oracle 9 and then a few months later we upgraded our user
and developers machines from Windows 95 to Windows 2000.
There are two users who use this applicatin 8 hours a day.  Somewhere
along these updates, the dbgrid started randomly freezing when they
are typing.  This happens 2-10 times a day.  Unfortunately they did
not start telling me immediately about the errors, so I do not know at
which upgrade the problem started.
I have error handling on the application so that if they get an
unhandled error that an email is sent to me.  I have gone through the
code and fixed every single error and this still did not help.
I have asked them to watch to see if there was any pattern, but they
can not see one.
I have asked our Oracle Guru to check to see if any oracle errors are
being generated, but they are not.
I had a trigger on the detail table that is connected to the dbgrid
and I tried disabling the trigger, but it had no effect.
I even went back to my Windows 95 and compiled the executable, but
they still got the errors.
I put some temporary buttons to refresh the query attached to see if
that enabled them to type, but it did nothing.
The only thing that helps them is closing the application and
reopening it then they are fine for a while more.

I know this is a really vague question, but I am running out of things
to check.
Can anyone point me in some new directions.

This is so frusterating.

Thanks in advance.

Karen