I know that if I'm on DH's computer using IE, instead of on mine using Flamingvixen, every letter typed will make the reply box jump up and down. IIRC, it's some setting that he doesn't want to change.
I solved it by copying the whole message from the reply box, pasting it into Wordpad, making my commentary, copying it again, and pasting it back into the reply box. It's an awkward workaround, but it keeps the problems from happening.