Is this site being hosted on servers you maintain locally or co-lo? What is your bandwidth cap and what server OS and Webserver is the site running on? I also notice you're using vBulletin 3.5.4, the latest version is 3.6.1, which you should be able to upgrade to for free if you have the (Owned License). Have you also updated / patched MySQL and PHP?
Not be be rude or anything, but it almost seems like the server is running on someone's Windblows box in an office somewhere connected to a DSL modem at best.
Oh, and to help in troubleshooting, I tend to get this error quite often when posting, after submitting the post: "Invalid Forum specified. If you followed a valid link, please notify the administrator". It seems to forget where it was, going back and refreshing often resolves this and sure enough the post goes through.