Pantheon site crashing every few hours

Hi,

I have installed pantheon on Ubuntu (lucid). But please note I have downgraded the php to 5.2.10 (karmic version) since the site was giving some errors with drupal 6.13.

Server softwares

Ubuntu Lucid
Apache/2.2.14
PHP 5.2.13 (cli)
Mysql 5.1.41-3ubuntu12.3
Varnish-2.1
Mercury 1.1

The site is working good.. but from time to time load increases tremendously and server just hangs. Seems like php connections are not getting closed.
This is from error.log

[Mon Jul 12 20:47:16 2010] [error] child died with signal 9
[Mon Jul 12 20:47:24 2010] [error] child died with signal 9
[Mon Jul 12 20:47:24 2010] [error] child died with signal 9
[Mon Jul 12 20:47:36 2010] [error] child died with signal 9

My server tuneables
/etc/apache2/apache2.conf

export APACHE_MAXCLIENTS="10"
/etc/apparmor.d/usr.sbin.mysqld

export APPARMOR_MYSQLD=""
/etc/default/tomcat6

export TOMCAT_MEMORY="128"
/etc/default/varnish

export VARNISH_MEMORY="64"
/etc/memcached.conf

export MEMCACHED_MEMORY="128"
/etc/mysql/my.cnf

export INNODB_BUFFER_POOL_SIZE="64"
in bytes (ie, 1Gb = 1073741824 bytes)

export INNODB_LOG_FILE_SIZE="1073741824"
export KEY_BUFFER_SIZE="8"
export MYSQL_MAX_CONNECTIONS="20"
/etc/php5/apache2/php.ini

export PHP_MEMORY="96"
/etc/php5/conf.d/apc.ini

export APC_MEMORY="128"

Could anyone tell me how i can make sure client connections are closed correctly. Due to this server load my site and even my server is going down. I think this happens after around 3-4 hours of running.

Regards