History of Problem:
1. Under images, I changed the second and third tabs - "highlights" and "options" - from theme default to display normal html: [url removed, login to view]
..this was done using 'Toolset' plugin (available through menu), which created custom html fields when I edit the page.
2. Issue one: preview button does not work for those new fields.
3. Issue two: sometimes <p> tags vanish from html field upon updating page.
This could just be issue with Toolset plugin setup, not sure.