Bug in 4.6.3 and 4.6.8
It sounds like you're experiencing a JavaScript-related bug.  This can happen for a lot of different reasons - some of which are tied to your setup (e.g. a conflicting Wordpress/Drupal module) or even your browser settings.

You should be able to troubleshoot this using the Troubleshooting JavaScript in CiviCRM advice.  Before doing that, I'd definitely follow choster's advice and test this on the demo site (https://civicrm.org/demo) to help isolate what might be causing the issue!