Problem with Master Template and exporting

ID CS 5.5. Version 7.5.3

Windows 7

Hi Forum,

I am applying a simple Master template to a 28 page document.

If I export to pdf using the pre-defined format for smallest file size or PDF/X-4 the export is correct:

If I export using the pre-defined PDF/X-1a or X3 I get some pages like this (Logo and Page-No. block on both sides):

This only happens on pages 8-9, 16-17, 22-23 out of 28 pages.

I have tried exporting to idml and reopening.

I have removed the master from all pages, checked for spurious elements and then re-assigned the master.

Drunk a lot of coffee!

No pictures go over the gutter.

I need the finished PDF to be in PDF/X-1a.

1.) Why does InDesign hate me?

2.) Can someone please suggest a cure?

Thanks for your time and consideration.