I also have this problem - at times it makes the board almost unusable.
It started about a week or so ago after having had almost 10 years with no problems. It's so bad with me that most times when I log in OK ("Welcome Urban Worrier"), then click on a link to one of the forums, I find I'm logged out immediately. Also, once logged-in, I can type a reply like this one, click on "submit" and get the message "not allowed to post, please log-in" (or similar).
Have tried clearing cookies, both by "my place", Ccleaner and Internet Explorer / Tools, all to no avail. Nothing has changed on the PC (no MS fixes, no new software installed - it's sooo frustrating