Black Cat
You will find the login requirements are somewhat fickle and changes from time to time.
Sometimes I can go for months and stay logged in, just for the last couple of days though I have logged in 5 out of ten times that I have visited the site. In the last year I would say that staying logged in works OK 80% of the time (This is with no changes to my PC or its configuration).
I visit about 10 forums and this is the only one that exhibits this odd erratic behaviour but I've got used to it so just accept it.
I used to play around changing settings, changing browsers etc, in an attempt to find the problem, nowadays I don't bother as I know it will cure itself in a few days.
Ian P