Either on this or another forum I have had problems posting if I've been writing and editing a post for a long time.
I would suggest that you preview the post every so often (now I don't know what that should be) just to let the forum program that you are there and will so want to post to the fourm.
If your going to do a long and complicated post another suggestion is to write and edit it in a word processor program (like Microsoft Word) then reopen or refresh the forum. Then copy the text from the word processor and past it into the compose reply box.