If you are logging on, and 'leaving' the iboats browser window open, but not actively viewing pages or posting, the system will occasionally log you out.
If you use your email notifications, and click a hyperlink in one to open the iboats forum, it might re-log you in each time you click a hyperlink.
I'd bet you just need to make sure Remember me is checked at the bottom of the login, you won't have the same trouble w/ Not Authorized to post. Try logging out, and relogging in and checking the Remember me.
