stability problems

Hello Newsgroup

I have stability problems with my Postgres 6.4 database. I am using PHP4.0.4
and get access to the db. Sometimes when about 10-20 clients are using the
website the postgres deamon shuts down and the message "no backend cache"
appears. The processor ist at 60%-80% and about 10-15 postgres processes are
on my linux machine. So what can i do ?

My postgres-command is: /usr/lib/pgsql/bin/postmaster -B
2048 -i -o -F -D/var/lib/pgsql

Thanks
Lars