There is a bug in the WordPress plugin for TinyMCE (which provides the more and next page tags. If you attempt to use an ampersand, greater than, or less than, each attempt to edit the source OR to save the file results in an endless string of ampersand-amp; (not shown right since this forum has the same problem).
The plugin filters out the ampersand character, but TinyMCE already does this, so the WordPress code just messes things up.
There is an easy fix:
Delete every line in the wp-includes/js/tinymce/plugins/wordpress/editor_plugin.js file that refers to the ampersand character:
content = content.replace(new RegExp('&', 'g'), '&');
content = content.replace(new RegExp(' ', 'g'), ' ');