I have noticed this as well and have found one thing that might be the cause.  It seems that if I'm preparing a post in a thread and someone else has placed a reply in the same thread before I do, mine disappears.

Lately, if I feel a diatribe coming on  I've gotten into the habit of copying my post to my clipboard before hitting the Post Reply button.  That way if I get blown out, I can just select to post again and paste my text back.