ShmemAlloc: out of memory

hi all,

    i need help on this.. i'm experiencing this problem too often and i need a
permanent solution.

    im using 7.3.4

.... config file

tcpip_socket = true
max_connections = 150
superuser_reserved_connections = 2

port = 5432
#shared_buffers = 45800
shared_buffers = 47900
sort_mem = 40000
#max_locks_per_transaction=160
max_locks_per_transaction=200

#fsync = true
#wal_sync_method = fsync

#
#       Locale settings
#
# (initialized by initdb -- may be changed)
LC_MESSAGES = 'en_US.UTF-8'
LC_MONETARY = 'en_US.UTF-8'
LC_NUMERIC = 'en_US.UTF-8'
LC_TIME = 'en_US.UTF-8'

... config ends

im having problems on our DB..

# update table_fan set active_flag='n' where num='12345678';

WARNING:  ShmemAlloc: out of memory
ERROR:  FreeSpaceMap hashtable out of memory


temporary solution:
  restart the DB and all things went back to normal. trying the same SQL would
result to an update..