[AccessD] Changing the ODBC Connection String to use UNC - Strange Results

All,

We have an Access 2007 application that connects to a FireBird Database
via ODBC.
This application has worked nicely for several months.

There is both a Test and a Production copy of the Firebird Database on a
server.

I have mapped one drive to point at the Production database and mapped a
second drive to point at the Test database.

There is VBA code that switches between Test and Production by changing
the DbName in Tabledef.Connect.

This works nicely with the mapped drives.

Recently, I had the idea of using a UNC instead of the mapped drives.  I
wanted to do this because more users need to access the Test database
and I wanted to avoid the need to set up mapped drives on additional
PCs.

I changed the VBA code to use the UNCs and I am now seeing inconsistent
results.

Sometimes the switch between Test and Production works, sometimes it
does not. (Error 3151 - ODBC Connection Failed).

Has anyone else ever run into this issue?  I have tried many tests and I
cannot understand why this is happening.

Thanks,
Brad