Phil, this something that has started happening to me in the past couple of months. It's not a big issue that bothers me but it is happening. I might be good for a few days and then I have to log in every time I get on for awhile, then it seems to go away again.
A couple of times, it happened again today, I log in and then click on "View New Posts" and it logs me off.
I didn't complain because I was sure it was something with my system, but if others are experiencing the same thing then something must be wrong.
Not going to keep me away though .