Continually needing to login as site admin

I have got a 3.5.1 site and about a month ago it started to force me to login every couple of minutes. Now it is forcing me to relogin almost everytime I pick an item from the dashboard.

I have not added any new plugins or themes and I have disabled the ones that are not being used.

• Has anyone else had this problem?
• Does anyone have any suggestions for what might be causing it or how I might narrow down the problem?

Thanks in anticipation of any assistance,
Petee Evans eMarkingAssistant.com