Hi, I just tried again. I installed Jetpack by finding it on the Plugins admin page and clicking on install. It claimed to install correctly, but then the site hung. I used Plesk to delete the Jetpack directory in the Plugins directory, and the site immediately began functioning again. This generated only a single line in the error log: “[Thu Feb 28 06:10:06 2013] [warn] (70007)The timeout specified has expired: mod_fcgid: can’t get data from http client” I have no idea what this means!
It sounds like the server is timing out in it’s connection to WordPress.com, maybe. Are your other 2 sites on the same server? Also, can you please try installing the Jetpack Compatibility Plugin so we can see some additional information from your site?