Everytime I visit fc (iPhone, or windows pc with firefox) I am already logged in or automatically logged in. Once in a while, regardless of which device I'm using, I won't be logged in when I visit the site. When I have to type my info in, it does the same thing to me as you; logs me out after posting something.
think i noticed the issue, i visit without the www. and when i post i think it redirects with the www. in there and the cookie settings are not synced since the site name has changed. i think thats it, thanks!