Which browser do you use?
When I went to IE10 (on Win7), I found that the browser did not handle secure sites well. I would access a site like FSM and get logged in, but then I found that every couple of hops from one page to another, I would get prompted to log in again. It happened here, at eBay, Facebook, Amazon, every forum I belong to. It happened regardless of any specific setting on any page to save my credentials or to keep me logged in.
I switched to using Firefox, and this did not occur, so it definitely seemed like an Internet Explorer issue. Also, I recently upgraded IE to 11, and found that the problem persists, though it's not quite as bad as it was with 10.
I don't see this with Chrome, either.