Edit page continually jumps to top while editing

While editing content in a WP page, the page continually jumps to the top, so I have to constantly scroll back down (or hit the PgDn key) to return to the part being edited. My workaround is to do as much editing as possible in Notepad, then copy to the WP page. But this isn’t always practical. Is there a way to keep the page from these spontaneous shifts?

Thanks,
Mark