Typically I've found this to be caused by logging into the same site in multiple environments. For example - widgetsite.com, dev.widgetsite.com, and widgetsite.dev. The cause seems to be ExpressionEngine getting confused by the cookies from one version of the site when accessing another. I've only found two solutions to the problem. The first is to clear cookies as you're already doing. The second is to use a different browser for each version to avoid cookie conflicts.
All of this, of course, only applies if you're referring to an environment like I'm describing. If not then this won't help you.