This has been a problem since I have been a member. Either while composing or responding to threads if any amount of time is spent creating the composition I find I am "not logged in" when I try to post the composition. For some reason the site monitor has not recognized that I have been typing the whole time and to recognize that as an "activity". I know there is an auto-save function but it does not always get everything that has been typed. It should not be that hard to have editing (typing) activity keep your login active.

Click the remember me box when you log in and that should eliminate the issue.

...and make sure your browser is accepting and not deleting cookies