APM my.policy hangs on blank screen

We've been having a few odd issues where /my.policy hangs up when a user goes to log into the system through the APM. In our access policy, we use an external login page, which may or may not be relevant, and under most circumstances, everything works. However, sometimes when the system redirects a user to /my.policy to start the session/login process, it just hangs. From the logs, the request makes it to the server, it just seems like the response never makes it back to the client. To get it working requires a refresh (usually non-cached (e.g. Ctrl+F5)), which effectively acts as a submit for the external logon page with empty values).

Anybody else seeing this problem or have any idea for a solution? It's often enough to be an issue in our testing before we make our move to a production environment.