There appears to be an issue with Internet Explorer 8 where if you type a post or reply that is longer than the height of the text box then the text box will scroll back up to the top. If you keep typing, it will produce a "jumping" scenario where the text will jump back and forth like the animation below.
It seems to be isolated to Internet Explorer 8 as it does not jump in Internet Explorer 9, Firefox, or Chrome. I know that most of the members of the board cannot install a new browser due to IT lockdowns so if you are using IE8, you can use IE7 compatibility mode which seems to solve the issue.
In the address bar there is an icon with a piece of paper torn in two (see figure below). Click the icon to enable IE7 compatibility mode. The setting applies to this domain only so it will not affect any other website you visit.