Google will usually only send one spider thats really efficent, its probably that pesky yahoo bot which is killing you.
Use robots.txt to slow the spiders, you can see what should be in a standard
vBulletin robots.txt and where to put it on your server, and then just adding something like
Code:
User-agent: *
Crawl-delay: 120
to the end of it will slow the bots. These changes probably won't take effect for 24hrs, as most spiders only download robots.txt file once a day.