Managed to figure it out, it was caused by the form not being able to find a common split point between the two side by side text boxes.
There is an excellent blog post about this problem at http://blogs.adobe.com/formfeed/2009/06/displaying_parallel_columns_of.html
I had tried his suggestions earlier but it turned out that even though both text boxes used the same font/size etc, having them vertically center align was causing the problem and making them vertically top align solved it.
Thanks also to Pgurrett about the version 8.0 compatability tip as version 8.0 does not support content splitting accross pages.