SMF 2.0* seems to do business differently than SMF 1.1.*
I too had this issue on any forum that used the new forum software.
It turns out that Internet Explorer has the problem as FireFox does not.
Also, IE also has trouble in the editor once you get to about the 25th
line down, FireFox doesn't.
In fact... I have completely switched over to Firefox and ThunderBird.
Joe