Seems like a certain script or something is eating up all of your resources, and most likely it takes about 2 hours for this to happen. Probably whatever is causing all of the zombie processes.
Since you are using cPanel/WHM, login to WHM and check this:
On the left menu, under the "Server Status" section, click on CPU/Memory/MySQL Usage
Take a look and see what user is using the most resources (whether it be mysql itself or your one vbulletin account) and it should tell you the scripts or commands that are using the most resources.
Otherwise from your info, nothing is terribly out of whack, other than what looks to be a script bombing out, creating a zombie, and this happening over and over again until the system comes to a grinding halt.