I have a user on my forums when logged in using the forums if he navigates the site back to the index using the logo in the top left or by pressing browse he gets automatically logged out. I'm able to reproduce it myself on firefox too.

Has anyone else experienced this? 

have u clear all caches in browser

Yes.

have you add any codes to your theme be for it started to happened...or any pluging try to disabled them and re-enable one by one to if that is the issue