If you have SSH/terminal access to the server (assuming you do by the above post), when the server begin to have severe issues, type in this to the terminal:
Code:
ps auxf | sort -rk 3,3 | head -n 10
Have you checked any access or error logs to your web server's vhost for the domain by chance? Have you checked for any long-running SQL queries? Are your forums apache processes running on the same as your SQL server?