I'm pulling the little hair I have left out!
On my server (CentOS 6) I have the default installation of Magento CE 1.8.1 and everything functions fine except for the USPS real-time shipping quotes in checkout. Half the time they appear and the other half the time there's nothing. I have debugging enabled and the times it doesn't return anything it's making the exact same request, but the response is NULL. I've even traced it all the way down into the code of Usps.php where the Zend_Http_Client is making the call. I simply cannot figure out why it's only USPS. Others, like UPS, grab the real-time quotes every time--but not USPS.
At first I thought it was my USPS account and I've tried three others since. It makes no difference. Now I'm starting to think it's something with my server but I have no idea what it could be. There aren't any scripting errors or timeouts but maybe my nginx and php-fpm don't like the Zend_Http_Client?
I'm completely stumped. Any ideas or suggestions I could try to narrow it down further?