"Sometimes yes, sometimes no" is exactly the symptom of a problem with the cookies. Three or four years ago, I located part of the problem: some of the links on this site were for kittycats.biz and some were for
http://www.kittycats.biz. Once they fixed that, the number of complaints dropped off significantly (in fact, this is the first I've heard of it since they fixed the links).
Perhaps you're using a
http://www.kittycats.biz bookmark? If so, re-create it correctly, without the www, and clear your cookies.