Highlight appearing around text unexpectedly on roll over

Hi,

A strange phenomenon has started to occur on some slides within a projects I am working on.

When the course is published or previewed and you roll over text on a slide a block highlight occurs around the text.

I checked States and effects to make sure there was nothing else causing it.

The workaround I have is to re-create the text box and put the text through notepad just in case there is something attached to it.

Does anybody know what may causing this? I would rather understand what is happening than re-working text boxes.

Thanks