Ok. this problem has been for a couple of years. Don't know why it started. Don't know if it can be fixed without deleting my account and starting over, which I would rather not do, so that's why I havent' brought it up until now.
The regular way of logging in does not "Stick". Says I'm logged in but when I go to post I am logged out.
To get logged in I go to the Search button and log in there, then to the forums then post.
Are you using the "remember me" option when you login? If you are try leaving that box unchecked and see if that helps. Unlike most forums that log you out after 10 to 15 minutes of inactivity SimHQ doesn't log you out for an hour.
I never use the remember me option. I log in and log out every time I use the site. Not using the remember me option may solve your problem but I won't guarantee it. I just know that I don't get logged out when I try to post, at least not yet. I have been timed out and needed to login again if I was idle for more than an hour though.