Weird DB Shutdowns

localhost[/usr/local/pgsql/data]# psql test dbuser
Password:
psql: The Data Base System is shutting down
localhost[/usr/local/pgsql/data]#

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I am getting weird DB shutdowns. Anybody experience this before ? Any ideas
welcome. What could this be?

I am running Solaris 7
/etc/system has the following shared memory setting:
set shmsys:shminfo_shmmax = 0xffffffff

test is a test database created for logins, etc.
dbuser is a user in the db. it's password is correctly entered. Sometimes
there are no problems but other times this failure happens.

Could this be something with persistent connections? What settings set the
length of time that a persistent connection will stay open?

Here are the startup settings (data/postmaster.opts):

-p 5432
-D /usr/local/pgsql/data
-B 64
-b /usr/local/pgsql/bin/postgres
-d 3
-i
-N 32