sometimes the re-loading of the #display div doesnt work
Can you clarify what you mean by that? What data gets displayed that indicates that the reload worked?
I can see a couple potential issues. 1. The call to $.post is asynchronous, so if that is doing something that will change the result of what gets displayed by display.php, then you have a race condition where display.php may be called before the post has finished. If it needs to be updated only after the post, then move that portion to the callback of the post call.
2. If the browser caches the result, then it's possible that it will just keep pulling the cached version of display.php. You may want to look into that to make sure load is not getting a cached version.