AFAIK, this happens when you are using the latest browser version which is not fully tested/compatible with the CQ authoring interface.
See the supported browsers for your CQ version.
Remember this list is only for the browser compatibility with the authoring interface. This is in no way related to the end user experience (published websites).
Also, happens sometimes because of cache issues. Try to clean up you cache and restart your CQ5 instance.
Sometime ago I got the same issue and by doing this, the problem is solved.