Bots causing extreme server load on WordPress

My server’s been plagued with random spikes in server load (to the point where I have to reboot or slowly log in and shut down and restart apache for it to recover). I finally figured out how to monitor things in Apache and it turns out that it’s happening when (I presume) spam bots are hammering at my WordPress archives.

These requests come from the same IPs during the attack/flood/whatever. Though when it happens again they’re different IPs. Today they’ve been hitting all my monthly archive displays (/archives/2005/08, etc). The IPs aren’t listed as being from any search engine.

If it were just a matter of bandwidth it wouldn’t be a problem, but this is taking out my entire server. Is anyone else dealing with this and could you point me in the direction of how to handle it?

I’ve installed the Bad Behavior plugin but I’m nervous about it blocking legitimate requests. Is there anything else I can do? I had WP-Cache on during the last time this happened and it didn’t seem to help. Thanks.