search and replace string from a script

[Root of all Evil]

Is there any way within ex (or some other text processing utility) to
access the nth occurrence of a pattern?  What I'd like to do is search
a file for the nth occurrence of a pattern and then change that pattern
but no others.  I've tried using ex:

ex -s FILE << QUIT
/STRING1/n s/STRING1/STRING2/
wq!
QUIT

but this only places me n lines after the first occurrence of STRING1.
Any ideas?  I'd like to avoid writing to a temporary file.

Thanks...

   Rich

P.S.  I use the Bourne shell.

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Rich Lenihan                 UUCP: mit-eddie!progress!rich
Progress Software Corp.      Internet: rich at progress.com
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