CakePHP session/auth logging out intermittently

I'm having reports and complaints from my user that they will be using a screen and get kicked back to the login screen immediately on their next request. It doesn't happen all the time but randomly.

I am using CakePHP and the Auth component, which seem to work well other than this issue

I got some feedback on the Cake forums once that this is sometimes caused by a 404 request that resets the session, i.e. if you have a broken image link or a missing favicon file. I have firebug open and there are no failed requests, so I ruled this out as a possibility, but the user is getting sporadically logged out. This seems to occur across browsers and operating systems.

Below is a summary of my config settings:
Security.level = high
Session.timeout = 1200 // this means my actual timeout should be 12,000 seconds
Session.save = php

I am really at a loss as to what is causing this issue...