Wow... that VPS should be plenty for your traffic. I've never seen this either. WordPress runs pretty well on pretty bad shared hosting. Your server should be pretty quick.
Centos 5.*? Centos 6? Php version? MySQL version? WordPress version? Apache or Apache 2? Anything else running like Suhosin? Do you know if the VPS is running any network monitoring tools or firewalls or anything like that?
Version issues can cause trouble-- just had some trouble myself with a MySQL database that was transferred from a Centos 5.5 to a Centos 6 VPS. I ended up needing mysql_upgrade due to the MySql version change.
I'm not sure this is a WordPress problem, though. I would try re-uploading all of your core WordPress files in case one of them is corrupt or missing. If you can, use clean file from a fresh download-- whatever matches your current version of WordPress. This does fix a fair few problems, however trivial the fix sounds.
Sorry about not being much help but this could be a tough one.