Symfony 2 AppCache behaving differently when under heavy load - http

Question

I am experiencing a strange problem with symfony 2. I have an app which has a count down timer on it (counting down hours minutes and days).
This minute by minute timer works perfectly 90% of the time, but when under heavy load, the timer got stuck and wouldn't change (presumably because it was serving a cached version):
We have been unable to replicate the error using a load testing script, it seems to only happen when there are lots of actual users on the site.
Our cache options are setup as follows:
class AppCache extends HttpCache
{
protected function getOptions()
{
return array(
'debug' => true,
'default_ttl' => 1 * 60 * 60,
'private_headers' => array('Authorization', 'Cookie'),
'allow_reload' => false,
'allow_revalidate' => false,
'stale_while_revalidate' => 2,
'stale_if_error' => 60,
);
}
}
The response that is being sporadically cached has the following header:
HTTP/1.1 200 OK
Date: Fri, 17 May 2013 09:36:55 GMT
Server: Apache/2.2.16 (Debian)
X-Powered-By: PHP/5.3.3-7+squeeze15
P3P: CP="CURa CUR NID ADM ADMa DEV DEVa PSAo PSDo OUR BUS UNI PUR INT DEM STA PRE COM NAV OTC NOI DSP COR"
Cache-Control: private
X-Symfony-Cache: GET /my-app/show: miss
Vary: Accept-Encoding
Content-Encoding: gzip
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: text/html; charset=UTF-8
Our Front Controller script file is as follows:
<?php header('P3P: CP="CURa CUR NID ADM ADMa DEV DEVa PSAo PSDo OUR BUS UNI PUR INT DEM STA PRE COM NAV OTC NOI DSP COR"');
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\ClassLoader\ApcClassLoader;
$loader = require_once __DIR__.'/app/bootstrap.php.cache';
require_once __DIR__.'/app/AppKernel.php';
require_once __DIR__.'/app/AppCache.php';
$loader = new ApcClassLoader('sf2', $loader);
$loader->register(true);
$kernel = new AppKernel('prod', false);
$kernel->loadClassCache();
$kernel = new AppCache($kernel);
Request::enableHttpMethodParameterOverride();
$request = Request::createFromGlobals();
$response = $kernel->handle($request);
$response->send();
$kernel->terminate($request, $response);
We haven't explicitly set any http caching on this action, or set anything to be cached apart from the fact that we are using the AppCache to wrap our kernel. Doing this definitely makes a massive difference to the performance of our site, as when we take it off, the load goes up very quickly during load testing.