Next time when you're logging in and it doesn't work, could you copy and paste the url of the page you attempting to login from and the page you end up at? You can paste the url right into your reply.
One thing I wanted to mention (although it doesn't sound like it's the problem) is if you use different computers you'll run into a situation similiar to this. The authentication process issues a "key" (which is stored in your cookie) that is verified every "x" minutes. If the key matches to the original, a new key is issued. If the key doesn't match, you're bumped out.
If you're using the same computer, it's not a problem. If you're switching between computers, everytime you login with one computer you're invalidating the key on the other computer.