Bob I get logged out too……after ages,though.
Since I used Firefox I no longer had to add sites to the anti-spyware trusted zones.Firefox seems to both ring fence websites from clearing my cookies and stop my anti-spyware prog zapping them as spyware.
I wonder if the next time you have a login problem you use the Tools>Clear private data ( or Control+shift+delete) and make sure that erases old unwanted cookies by going to Tools>Options>Private data>settings
tick cookies & cache and untick saved passwords
I doubt that old modelboats cookies are a problem but the above measure wipes the slate clean.
3-rd party cookies setting is not relevant in Model Boats
I only temporarily permit third party cookies when I visit a new site and 3rd party cookies need to be allowed (i.e. hotmail uses live.com) .I would then enter the 3rd party site in the exceptions list to allow cookies from that site and then I switch 3rd party setting back to off.