Random wordpress logouts

Hi All

I have had my site up and running fine for months now and all of a sudden I keep getting random logouts as I am navigating through the dashboard, updating posts or pages and I am being sent back to the wp-login.php page for no reason?

Has anyone else experienced this and have any ideas on how I can fix this?

The site has not had any work done down it for at least 2 weeks (plugins/upgrades etc)

Thanks
Matt