I had this problem with a post that had a lot of embedded video content. It seems to be that WordPress prefers to break long content over more than one page.
SO – you need to use the <!--nextpage-->
code to split your post into manageable chunks (see Codex).
In the HTML view, just add <!--nextpage-->
at the cursor wherever it works best for a page break. Simple and worked for me…
There’s a discussion about this here too.