Finally! Figured out how to fix the jumping text-box when your post gets lengthy (for IE8). Click on compatibility view (it's the broken piece of paper at the right end of the address bar, right beside the refresh arrows and the red X for stopping a site from loading, just to the left of the search box. Click on that so it's lit up light-blue and that fixes the problem. Hope this helps!
Also, here's a thread discussing it if an administrator of a site is having a problem with it, I think:
http://www.simplemachines.org/community/index.php?topic=376321.0