It appears that Internet Explorer 8 has an issue with the posting page (posting.php?mode=post
The problem is with the main text box for the message, once it reaches a certain number of lines, it does not want to display the line you are typing on as you type. To put it another way, once you are past a certain number of lines, the text box does not remain scrolled so that the line you are typing on remains visible. As you type a letter it jumps down to that line, but it then jumps back up, hiding the line below the edge of the text box. You can scroll down and view the line as long as you are not typing, but if you try to type, it starts jumping around again.
It looks like turing on "Compatability View" in IE8 for this page fixes the problem, but it should probably be looked into.