After a few seconds my page becomes 'not found' or changes to another page.
You may have accidentally turned on the 'redirect' option. That option allows you to have a web page switch to a different page after a specified number of seconds.
To turn this option off, go to the problem page (in SiteSpinner or Web Engine), select the menu 'Page' > 'Page Editor'. Then go to the 'Links' tab and Un-check the 'Redirect to New Location' check box. Press OK.
That's it. The page should no longer change unexpectedly. You just need to re-publish your page.