Help diagnosing slowness - hacks the problem?

[Sadie Frost]

I've been racking my brain over this for a while and my limited knowledge is clearly not enough so I'm hoping someone will have some advice for me.

I'm on a shared server, and I run multiple vb board sites on it, all 3.03 at the moment. One of them has started to run very slowly, often coming up with page not found errors, when the other boards run just fine. I usually don't have more than 20 users on at one time, some of those guests/spiders (though I do have a robots.txt that is supposed to be blocking them but whatever). Just now I have 12 users on, and the server load was over 22. I've repaired and optimized the tables, have gzip running, don't allow guests to view attachments. I've looked through the logs and not seen anyone hotlinking much.

I don't know much about this, but I'm thinking that if they're all on the same server, it's not the server load that's the problem? If that's the case, is it likely to be a hack I've installed, as this particular board is more heavily hacked with items like the store and pets, vbchat? Would the best way for me to diagnose this be to start with clean files and add back the hacks one by one?

The site in question is http://www.buffy-boards.com

Any ideas would be greatly appreciated!