Definitely not bandwidth related. The bandwidth graphs show peaks and valleys. If it was bandwidth or capacity there would be flat plateaus.
If the forum isn't meant to be searchable, just throw in robots.txt and tell them to go away (or I can forcefully block them at the web server level), if it's meant to be searchable, then the "search friendly" links should be used so the search engine bots don't get lost. There's also a sitemap XML that can be built.
But realistically, the search engine bots shouldn't be causing 500 errors unless something in the page is causing excess processing time (eg repeatedly spamming the search button... which they can't do because of the captcha) So this goes back to something that isn't part of the stock install possibly being the culprit.







