We've started having a problem that's been reported to me (and experienced myself). The forum seems to be logging people out at random times. Sometimes it's while browsing, but other times it's in the middle of a post.
So my first suggestion is that you save a copy of your post BEFORE you click the "Submit" button. The easiest way to do this is to select the entire text with your mouse and then copy it to the "clipboard" (typically with the Control-C key combination). Then if the submission doesn't work, you can paste the clipboard contents into an editor to save them to try again.
Also, I'm trying to measure how widespread this problem is. So if it's happened to you, please post here to let me know.
Thanks, and I'm sorry for any inconveniences.
Bob Kuczewski