The "#12" (or whatever) at the top right of your post is a link to that individual post and contains the post number in the URL. Once you click on the first link you see a second one in approximately the same position, "Thread: Posts still disappearing !" Clicking on that second link takes you to your post again, but this time with the rest of the thread around it. That's the page I bookmark whenever I want to refer to a particular post again for whatever reason. I think (but I'm not sure) the URL is the same as for the page you see right after you click "Submit". [I just checked with this post and yes, it is -- so you can bookmark right after posting if you want to.]

Once in a great while I've gotten a post the way I wanted it in Preview, then gone away without clicking Submit. Once in a great while, too, I've clicked Submit and eventually gotten a browser timeout instead of an actual page. Could either of those have happened to any of your missing posts?