PHP :: Flash - Post Variables Go Missing For Form?

Sometimes, multiple times a day in fact, users of my web application are submitting a certain form which has about a dozen form fields, half of which are hidden fields, and half of the $_POST data is simply not present in the processing script. Note that the fields that are not present are at the very bottom of the form. I know this because this raises a fatal error, and an email is dispatched to me which includes the post data. And of course, neither I nor any of the developers on my team can reproduce the problem. Flash is involved in the process, as I'm using a library called Uploadify to display a progress meter. Here is the flow...why some of the post data would be getting wiped out?

-User visits edit screen for a page in the CMS I am using.
-Record id for the page is put into a form as a hidden value.
-User clicks the Uploadify browse button and selects a file (only single file selection is allowed).
-User clicks Submit button for my form.
-jQuery intercepts the form submit action, triggers Uploadify to start uploading, and returns false for the submit action (manually cancelling the form submit event so that Uploadify can take over).
-Uploadify uploads to a custom process script.
-Uploadify finishes uploading and triggers the Javascript completion callback.
-The Javascript callback calls $('#myForm').submit() to submit the form.

This happens on multiple browsers (Firefox 3.5, 3.6, Safari, Internet Explorer 7, 8) and multiple platforms (Mac OS 10.5, 10.6 and Windows XP, 7).