Have you checked for any bottlenecks on gateway hardware? Switches/Firewalls/Routers? Do you have the OS X firewall enabled? If so, does this problem persist if you turn it off?
If you use ServerAdmin then go into the Web > Settings >General tab. You got the "Simultaneous connections" and other settings that will determine how often and under what circumstances Apache will spawn sisters and daughters. I think the default was 500/300/1/5/1. You may also want to checkout the "Persisitent connection" setting.