Jeff,
First, please check your setting for
default notification upon replies in your User Control Panel:

- 2014-02-25_09h31_50.jpg (28.9 KiB) Viewed 18845 times
Make sure it is set to No. If it is already set to No (I think Keith has done this for you) it may be an idea to set it to Yes, save, and set it to No and save again, just to be certain.

(This is a per user setting in the board's database, it is not a cookie).
Then, make sure that the checkbox at the bottom of a forum topic it says "Subscribe topic" (if it says "Unsubscribe topic" you have subscribed to the topic).
You may then also want to check the similar checkbox on a forum's index page, it should say "Subscribe forum".
(Again these are personal settings per forum/topic in the database, not cookies).
That should fix your "problem".
Forest,
As I explained elsewhere, the automatic login *is* a cookie that unwantedly may expire. I am experiencing similar behaviour on and off. There really isn't much I can do about it. Sorry for that. You could clear your cookies but the cure would likely be worse than the disease...