If you can get the document onto a Windows machine with Word 2007 or later you can use Word Toolkit for Advanced Management of Word Headers and Footers to figure out what is happening.

Otherwise, if you can post a portion of the document where the problem pops up here, someone can take a look at it.
How to attach a screenshot or file in this forum.

I have no idea why it changes upon closing/reopening.
I suspect either a poorly-written Add-In or that you have the document set up to update styles from the template upon opening. Those are guesses and probably not even close to accurate. They are things that can cause problems that occur when a document is reopened.