I saw a moderator suggest to you to select the 'remember me' option so you don't get bumped off. During the time you're writing (especially a detailed post that you are obviously taking time preparing), you must be 'timing out'.

Also, before you try to 'post', highlight and copy your work...just in case your log-in session ends, you won't have to retype it. Or do like Rosie suggested and type your thoughts on notepad or other format that can be saved.

Hoping these suggestions help and I'm looking forward to seeing your post.