why get 'not logged in'?

I log in. Get message ‘you are logged in’ etc. I go to a thread and type in a reply and hit button. I then get ‘you are not logged in’ screen. No matter what I do after this my message goes POOF into never-never-cyber-space land and disappears foreaver. What am I doing wrong?

This must be happening randomly, as you have nine posts on the boards. Am I correct that it doesn’t happen all the time?