intermittent very slow page loads – where to start?

I’m getting intermittent very slow pageloads (~20 sec or more) with a site I’m developing at https://new.chnnyc.org . It’s hosted on a VPS with 96MB ram allocated in php.ini.

I’ve read up on the various cacheing and optimization options, but something seems to be wrong here at a more basic level. I’m trying to track down the problem — any suggestions on where in the stack I should look for issues? Or tools I might use to track down the issue(s)?

Thanks in advance!