A quick way to see if lowering the maximum connections per ip will help.
/sbin/iptables -A INPUT -p tcp --syn --dport 80 -m connlimit --connlimit-above 10 -j REJECT --reject-with tcp-reset
# save the changes see iptables-save man page, the following is redhat and friends specific command
service iptables save
Run from the ssh command line. Find MaxClients setting in httpd.conf to see what it is before changing if you want.