Run Away Apache - Can Not Find Cuase

I am having a problem with my web server where apache is eating up all memory AND all swap. For example last night I left a SSH session open to my server running top sorted by memory usage and this am the server was down again, I was unable to reach it. I had request a reboot, but on my screen running top I had...

06:21:16 up 24 days, 2:47, 1 user, load average: 179.43, 137.53, 75.9
Cpu(s): 1.8%us, 9.4%sy, 0.0%ni, 0.0%id, 86.6%wa, 0.3%hi, 1.9%si, 0.0%st

It also reported 8mb of RAM free, and 0K of swap free. It was sorted on memory usage and my 17" monitor was filled only with httpd on that list, top entry said 385mb. This will happen with out fail fairly quickly if I do not intervene. Right now I have a cron script that runs ever hour and issues a

/etc/init.d/httpd restart

And that works until Apache runs out of semiphores, then I have to do a

/etc/init.d/httpd stop
for semid in `ipcs -s | grep apache | cut -f2 -d" "`; do ipcrm -s $semid; done
/etc/init.d/httpd start

for it to recover. Right now after the reboot, and 30 mins past the last httpd restart I have...

top - 11:33:05 up 2:00, 3 users, load average: 0.17, 0.12, 0.09
Tasks: 126 total, 1 running, 125 sleeping, 0 stopped, 0 zombie
Cpu(s): 7.1%us, 1.5%sy, 0.0%ni, 90.5%id, 0.8%wa, 0.0%hi, 0.0%si, 0.0%st
Mem: 960656k total, 785560k used, 175096k free, 40512k buffers
Swap: 1927792k total, 0k used, 1927792k free, 216472k cached

So all looks fine. Eventually that free number will fall to nothing, and the swap will start filling up. I do not see anything in the logs that is all that helpful. After a httpd restart I will have a lot more free ram.

Server is:

OS: CentOS 5.x x86_64 w/ PLESK + Power Pack

Hardware: AMD 64 Athlon 4200 DC/1024MB/160GB SATA

(if I could do it over I would not put PLESK on, turns out to be a PITB, my other servers do not have a control panel and all run fine... )

Apache is:

[root@www log]# rpm -q httpd
httpd-2.2.3-7.el5.centos

The primary function of this server is to run a web forum based on Invison Power Board, and I have kept up with their patches.

I have never had this problem on any server I have run.