Bouncing Add New / Edit Post

When I enter text into the Add New / Edit Post area of the Site Admin, the screen (not the cursor) keeps bouncing to the top of the input box when the content extends beyond the last few lines of the box. So tryng to input text in both Visual and HTML is hard work because the cursor stays somewhere at the place I’m typing, and the screen shows me the first 9 or 10 lines. Lots of scrolling down to see what I’m typing.
Does anybody have a solution for me?