not nescecarily. Not all cookies are the same, so the problem may lay there.
As you noticed in PM

you are definitly NOT on a static IP (unless you use several computers to post from). I connect to the forum from several machines in my network cluster, and all autolog me in (because my userdata and my cookie are on the server, and because the IP showing for me is my WAN-IP (AFAICT), but I had to relog when I used the UMTS connection.