[Chris8]

It looks like you are on shared server and others that share your server cause too heavy usage which your server is not able to handle. Another explanation might be that your forum/website is overwhelmed by bots crawling your forum/website. In a first case you can do nothing but to move somewhere else, in a second case you could review your website logs and then kick off rogue/bad bots out of your site using for example htaccess. If it's the problem of shared account and your server then move to some better hosting company that will ensure that you have dedicated resources and other server users cannot impact your account performance so everything will work smooth. You could also move to completely dedicated server but this is more expensive solution ($50/month or more depending on your needs) and I'm not sure if a small forum really requires it.

As per vbulletin performance itself there are a few things to enhance it, but if I good understand it was working smooth before so I guess the problem lies somewhere else and tweaking your site would not change much.