AIX 5.3 Out of Memory Error - 64-bit

Hi All

I'm looking for some help with regards a postgres 64-bit install I  set up on a IBM AIX machine.

I used the following install options whien I installed postgres

BINDIR = /usr/local/pgsql837-64/bin                                            
DOCDIR = /usr/local/pgsql837-64/doc                                            
INCLUDEDIR = /usr/local/pgsql837-64/include                                    
PKGINCLUDEDIR = /usr/local/pgsql837-64/include                                 
INCLUDEDIR-SERVER = /usr/local/pgsql837-64/include/server                      
LIBDIR = /usr/local/pgsql837-64/lib                                            
PKGLIBDIR = /usr/local/pgsql837-64/lib                                         
LOCALEDIR =                                                                    
MANDIR = /usr/local/pgsql837-64/man                                            
SHAREDIR = /usr/local/pgsql837-64/share                                        
SYSCONFDIR = /usr/local/pgsql837-64/etc                                        
PGXS = /usr/local/pgsql837-64/lib/pgxs/src/makefiles/pgxs.mk                   
CONFIGURE = '--prefix=/usr/local/pgsql837-64' '--with-pgport=5422' '--enable-thr
ead-safety' '--enable-integer-datetimes' 'CC=gcc -maix64' 'LDFLAGS=-Wl,-bbigtoc'
CC = gcc -maix64                                                               
CPPFLAGS =                                                                     
CFLAGS = -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Winline -Wdeclaration-a
fter-statement -Wendif-labels -fno-strict-aliasing -fwrapv                     
CFLAGS_SL =                                                                    
LDFLAGS = -Wl,-bbigtoc -Wl,-blibpath:/usr/local/pgsql837-64/lib:/usr/lib:/lib  
LDFLAGS_SL = -Wl,-bnoentry -Wl,-H512 -Wl,-bM:SRE                               
LIBS = -lpgport -lz -lreadline -lld -lm                                        
VERSION = PostgreSQL 8.3.7  

And the spec of the AIX machine is a follows.

AIX 5.3
2x Power 5 CPU's
6GB RAM

If I run postgres with the standard postgresql.conf install then all queries run (albiet sometimes slower than i'd expect).  However if I start to change some of the memory settings for example shared_buffers, temp_buffers, work_mem, maintenance_workmem above 128-256MB and run a large query or create a large indexes i get the error below

Out of Memory
Failed on request of size 167873 (this value will change)

Any thoughts or ideas would be greatly appreciated

Thanks
Fran                       
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