browser tries "localhost" instead of remote IP address

we run some simple perl, cgi scripts against a MySQL database located on an in house server running apache under Windows server 2003.

Some remote users (with Firefox and IE7) have started to see localhost in the address bar instead of the IP address after a form submission.  
e.g.
1. form presented on browser
2. enter search data
3. press submit
4. returned URL = http://localhost/cgi-bin/abc_change.cgi

instead of http://12.34.56.78:8080/cgi-bin/abc_change.cgi
(actual ip address etc changed)

The problem is not consistent i.e. sometimes it happens sometimes it does not.  Recently it was happening every other time i.e. try again and it is OK.  Using the back button and re-submitting seems to work reliably too.

Some remote users cannot reproduce this bevaviour.

Any ideas?
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