Hi Pete,
Unfortuantely, Maintanance Level 03 of AIX 5.2 brought in certain
backward compatibility issues that changed the API for programs like
nmon that use the libperfstat library. nmon now checks for this problem
and stops rather than display invalid data.

There were some other problems with ML03 and I generally recommend
customers upgrading to the current ML. The issue is fixed in ML05 and
ML06. If this is a production machine I understand that you may be
reluctant to update and can only do this after testing.

As a short term solution you should be able to run nmon_aix51, although
some stats are not available in this earlier AIX version. nmon_aix51
uses an even earlier libperfstat API which still works on ML03.

I now recommend nmon questions go on the Performance Tools Forum
website, so that others can find and benefit from the answers. Take the
Performance Tools link at the bottom of
http://www.ibm.com/eserver/pseries/community/

Sorry for the problems you are having, thanks Nigel