Found the problem so figured I'd post it as it may help someone else.
I had created a custom preloader which contained a dynamic text field (verdana). So that the text field updated as the app was loading I'd embedded numerals and the % sign.
However I hadn't embeded and uppercas or lowercase letters.
So that seemed to cause flex to behave strangely whenever verdana was used.
changed the dynamic text field to embed uppercase / lowercase and numerals and all is well.