It sounds like your browser has a mind of its own!
Every computer is different, but getting to the bottom of this sort of thing can be tricky. If clearing the cache, cookies, and temp files do not work, you might want to try temporarily disabling any virus or firewall software. Some of these programs, while beneficial, can attempt to override your browser's behavior and handling of cookies, etc.
If that doesn't work, you can also try re-installing your web browser or trying to use a different browser. I personally use and recommend Mozilla Firefox
BTW: I merged your threads and moved them to OT.