I am running a PHP site that uses Ajax and jQuery as well. The site will run fine for quite some time, and suddently my pages (and ajax-retrieved sub-pages) comes back with
This is a PHP issue somewhere. You could spend some time narrowing down which function you're using that is causing the problem. I would instead upgrade to a newer version of PHP. If still no luck, try a slightly older version. There have been significant changes with version 5.3.2.