Might be a daft question but you have clicked the 'remember me' button where you sign in (I'm thinking you have but thought I may as well ask it anyway)?
I've been having the same issues couple of times a day.
Have followed edds advice and cleared out my cookies but hasn't made a difference, just given me serious bloated feeling, but they were good nom nom nom
there will be a general inactivity logout time set, is it logging you out as your browsing the forum or when you click on say another thread after 5 mins of inactivity?
Sounds like your browser is rejecting cookies maybe? check the security settings because if your blocking cached login info a refresh with not remember the details, prob best to try a reset of the browser settings and/or an alternative web browser to see if it still happens.
Every day for months i have to sign in whether on phone or laptop. Even when I tick remember me. Use to think at first someone was trying to access my account but its daily