First, thanks for running CH. I've learned a lot ( and relearned a lot).
I'm curious how a CPU could run at 400%?
You're more than welcome, it's a pleasure running the site. This system is a linux based system and operates a little differently than what you may be used to in Windows, honestly doesn't make that much since to me either how something can surpass 100. However, each daemon (process) running on the system utilizes it's own percentage of the CPU, each of these processes can surpass 100 themselves and having dozens of httpd (a process that allows a web page or application to be processed by Apache, which allows you to view a web page) the CPU can quickly max out if something else is taking up a lot of the processor, which in my past experience has been a MySQL (database to run forums) getting stuck.
When the MySQL gets stuck because Computer Hope is averaging about 25-30 http requests a second and visitors don't stop visiting the site, things get backed up and the CPU percentage simply increases until it's so busy it can't anymore.
Normally if the CPU isn't getting hammered too much you can SSH into it and kill or stop the process that's using up the CPU, much like you'd do with ALT + CTRL + DEL. However, because the CPU was beyond it's max (normally wouldn't happen if I was able to look at it sooner) the only real solution was to reboot the machine.
Hope that helps give you a little better understanding of it.