It appears that, at the times that this has happened, most of the traffic on your site was from search engine bots and the automated limitations kicked in due to the speed and number of rows ready when they crawled your forums.
We have made modifications to your robots.txt to limit crawler from being able to make enough requests to cause a problem such as this. Should you need more information on this you can find more information on how to do this yourself at
http://www.robotstxt.org/robotstxt.html, or if you have any specific questions, please feel free to reply to this with them, and we will be more than happy to help you.
Unfortunately, if the bot hitting your site is one of Google's Bots, you will need to make modifications to the crawl rate through Google's Webmaster tools, so that it no longer causes an issue (
http://www.google.com/support/webmasters/bin/answer.py?answer=48620)
We also noticed that you are running a vBulletin script. There are some optimization techniques that you can find for this here
http://www.vbulletin-faq.com/optimize-vbulletin-server.htm. This should help you with caching and database optimizing that should increase performance while limiting server usage.
This should help to alleviate some, if not all, of the issues that you are seeing regarding this issue. However, if you do continue to see these issues please let us know with as much detail as possible, such as specific times and frequency, so that we may assist you in finding a resolution.