DB Stopped Choosing Index

Have an issue where after several months, our Oracle database has stopped choosing an index.

Here's some background information:

For the sake of simplification, the table has two key fields, SITE and ID, and the table is partitioned by SITE. Since the query is doing an ORDER BY ID DESC, we had to create a function-based DESC index as well as a regular index to convince the database to choose the index:

INDEX_1: SITE Asc, ID Asc
INDEX_2: SITE Desc, ID Desc

So when the index is chosen the showplan displays INDEX RANGE SCAN DESCENDING on INDEX_1

Now, after several months where the index was being chosen for all 4 SITEs, the index only works for 3 of the 4 SITEs.
And for 1 of the SITEs it shows 'TABLE ACCESS FULL'.

Strangely, we created another instance of the database (a byte for byte image) for troubleshooting purposes, with the same database engine version (but different OS version) and the index is being chosen for that new instance. Of course there's an explaination for this, but it's beyond my comprehension as to why you have an exact image with a different behavior.

Here's what Oracle version:
Oracle9i Enterprise Edition Release 9.2.0.7.0 - 64bit Production
PL/SQL Release 9.2.0.7.0 - Production
CORE9.2.0.7.0Production
TNS for Solaris: Version 9.2.0.7.0 - Production
NLSRTL Version 9.2.0.7.0 - Production

By the way, HINTS are not an option since the application that executes this query doesn't allow us to modify the SQL.