Apache hangs once a week or so.

I have a server that is just bugging the heck out of me. Running CentOS 5.5 with Plesk 10.1. At least once a week, Apache just hangs. I've looked through the httpd error logs and nothing jumps out at me.

I normally just restart the service and everything goes fine from there. Any thoughts on where I can look as to how/why this is happening?