See:
keep getting logged out
If you have cleared your cache and temporary Internet files and it still isn't working properly then consider re-installing your browser. Another thing to try would be if you are coming in from a bookmark do a CTRL + F5 straight away so that you get a fresh update of the page.
It sounds as if to me your computer is storing information (via cookies) for too long and either needs a clear out or your settings need a tweak.
Let us know what you find out.
