Using FrontPage can be problematic. It's probably gotten a little better in newer versions, but it is well known to generate inefficient and incorrect HTML, so fixing your pages and then loading them back into FrontPage can cause FrontPage to reintroduce problems.
If you are having problems when you "fix" your HTML then you're probably not fixing it correctly. If it is fixed correctly, then it should display correctly. To use CSE HTML Validator effectively, one must know some basic HTML, including nesting. Nesting simply means that if you use the "i" start tag then the "b" start tag, you must use the "b" end tag before you use the "i" end tag. In other words, the most recently opened tag should be the first one closed.
Hope this helps. Don't give up!