Not sure on what may be causing this Craig, but have you tried clearing your prefs and caches.
Thanks for the suggestion Javed, unfortunately that didn't fix it. I have however noticed that it only behaves this way in Overprint Preview.
What is the exact version of InDesign that you are using?
I believe you should be on 9.1, this issue existed in that version and has been fixed in 9.2.1
I would recommend that you upgrade InDesign CC to at-least 9.2.1
Updated and the problem is fixed, thanks Javed